Travel by recreational vehicle, despite the increased cost of gasoline, has become an important way of vacationing.

But you don't have to buy one. In addition to the estimated 8.5 million RV's registered to private owners in the United States, there is a sizable pool of RV's - campers, trailers and motor homes - for rent in the United States and Canada, as well as in Europe, New Zealand and Australia. Germans and Britons are big customers for rental RV's in the United States.The snowbird season is the next-to-biggest time of the year for recreational vehicle travelers.

Retired people living in the northern tier of the United States and in Canada unplug their coffee pots, put them in their camper trailers or motor homes and start driving to warmer weather.

The head of an assistance service that copes with medical emergencies of people on the road says he virtually tracks the weather by the area codes of calls for help. He said he gets a vision of motor homes down on their haunches like rabbits ready to streak for Orlando or Arizona at the drop of a snowflake.

By now, he says, snowbirds are resting in the sun on the east and west coasts of Florida, Southern California, South Texas and around Phoenix and Tucson.

In the main, snowbirds own their vehicles because they want to use them year round.

Younger families with tighter budgets and people thinking about buying an RV will probably want to consider rental.

Rentals are continuing to rise, as they have since 1980, sometimes by as much as 50 percent in a year.

Although rentals are not cheap and the vehicles use a lot of fuel, daily costs for a trip may be lower than renting a car and staying in hotels.

The savings, clearly, comes from doing a lot of your own work: cooking, bed-making, driving, cleaning, using the coin laundry.

Prices vary by area, length of rental, size of vehicle, length of trip and distance traveled. Demand also affects price. Here are some rates from the 1990 edition of "Who's Who In RV Rentals," published by the Recreational Vehicle Rental Association.

In Orlando, Cruise America offers 19- to 24-foot motor homes for three to five for $385 to $980 a week. Also in Orlando, Go Vacations offers 18- to 27-foot motor homes to sleep four to six for $161 to $826 a week. In Atlanta, Holiday RV Superstores quotes a 22- to 26-foot motor home for four to six at $548 a week.

In busy seasons most outlets have minimum rental periods, as few as two days and as many as seven.

Rentals do not include bed linens, pillows and blankets, pots, pans and plates, which are available for an extra cost, either by the person, perhaps $30 each for the whole rental, or for $50 to $100 for everything.

Most rentals allow 100 free miles a day, with a per-mile charge after than, and a basic collision damage waiver.

Rental involves a security deposit, possibly $500 for the vehicle, a cleaning deposit of $50 and sometimes an added deposit of $50 for sanitizing.

Bob Strawn, the executive vice president of the Recreational Vehicle Rental Association in Fairfax, Va., is blunt in sorting out the duties in an RV rental.

Strawn, who for 19 years was an RV dealer in Denver, said that the dealer must say to the renter:

"Now you are going to spend the next two or three hours listening to me about how this works. If you do, you will have a wonderful trip and come back again. If you do not, you may have an unpleasant and expensive experience."

Then the dealer should start by showing the customer how to put in oil, how to read the gauges and how to start the generator.

Customers, he said, should be sure they understand the vital parts of a big machine that includes plumbing, water supply, propane gas, two electrical systems and a sewage pumping system, just to name a few.

All of this rules out arriving at the airport at 4 p.m. and taking off that night, as one might with an auto rental.

Strawn said that sometimes Europeans acted confident about understanding the instructions and wanted to hurry away but discovered later, "usually at 3 a.m.," that their knowledge was flawed.

Strawn's organization includes 912 of the 3,800 RV dealers in the United States, but he says it is a purely voluntary group and membership is no guarantee to a consumer.

The small type on the individual contract will specify what the insurance covers and stipulate what the traveler should do in the event of problems. He said most rental vans were less than three years old and would be sold at 50,000 miles.

Failure to add oil or to understand how many hot showers are possible in a day, he said, tends to be the cause of difficulty rather than bad maintenance.

A dealer rents an RV 14 to 16 weeks a year, he said, with some very energetic dealers getting 22 to 26 weeks. He said that winterized vans, "mobile ski chalets," had longer seasons, and that sometimes dealers and hospitals enabled families to rent and live in a motor home near a relative who was getting treatment.

The Recreational Vehicle Rental Association publishes a good brochure, "Rental Ventures, How to Arrange to Tour North America by Rental Motor Home."

It comes with the directory of 100-odd dealers that rent in 37 states and four Canadian provinces, and costs $5. The new edition of the directory will be published in February.

RVRA, 3251 Old Lee Highway, Suite 500, Fairfax, Va. 22030.

A 1988 University of Michigan survey of 2,085 households is the source for the figure of 8.5 million recreational vehicles registered in the United States.

The survey also found that one out of 10 vehicle-owning families had at least one recreational vehicle, and that 2 out of 10 were interested in renting an RV for a coming vacation.

In Canada, estimates by Transport Canada and the RV association there put privately owned RV's at 500,000 to 750,000.

In the United States, according to the Michigan survey, 44 percent of the owners are 55 years of age or older; 39 percent are 35 to 54 years old. The remainder are owned by people 18 to 34.

The price of an RV varies widely: a camping trailer that unfolds into a tent may cost $2,000; a travel trailer may be $12,600. The industry association says that a luxury motor home costs an average of $59,000, with smaller models beginning at $26,000.

A standard motor home of 20 to 22 feet, sometimes also called a mini-motor home, sleeps up to five people, as does an intermediate model of 23 to 24 feet. A large model, 24 to 26 feet, sleeps six and a deluxe model of 29 to 31 feet sleeps up to seven.

The big motor homes used to give only 4 to 5 miles to the gallon, but David J. Humphreys, president of the Recreational Vehicle Industry Association, a manufacturer group, said in an interview that improvements in weight and aerodynamics had brought this up to 10 to 12 miles to the gallon.

Humphreys said that he expected 1990 sales, affected by waning consumer confidence in the economy, to be 7 percent below the 1989 sales figure of 395,700 units.

A new agency in the United States has linked up with a British company and is offering RV rentals in England, usually for those who travel in caravans.

The company, Britannia RV Rentals, lists rates of $760 a week in January for a vehicle with two double beds to $1,890 for a week in August for a vehicle with three double beds. It also has the smaller van conversions. The price includes pickup from Heathrow Airport in London, the 15 percent value added tax and a basic collision damage waiver.

Mandatory European Touring Insurance for trips outside England, $109 for two weeks, is also not included. The British company is U.K. Motorhomes Ltd.

Britannia RV Rentals, Box 1333, Jonesboro, Ga., 30237; (800) 872-1140.

There is also a clearinghouse for families with RV's who want to swap with European families who have RV's.

Like programs for home exchanges, this has become much more informal because the operator discovered he was losing money on the plan; he now only circulates the information and leaves the arrangements to the parties.

Bill Topping, who organized International Camper Exchange, will send an application form to anyone who sends a stamped self-addressed business envelope. When the form is returned, along with two or three envelopes each bearing 45 cents in postage, he copies it and distributes it to contacts in the countries that interest the applicant.

International Camper Exchange, Box 947, North Bend, Wash. 98045.