Renting a recreational vehicle proved not to be difficult, but we quickly found out that trips must be planned well ahead of time. There are only a few companies in the Washington area that rent RMs, and only larger, more expensive vehicles were still available by the time we decided to go. The smaller sizes-say, up to about 24 feet-must be reserved many weeks in advance.

Fortunately, a firm a little further away, Fredericksburg RV Rentals, still had available a 23-foot Tioga Arrow Equally fortunately while its location was 50 miles south of Washington, it was on our way. On the other hand, since we did not want to drive 100 miles just to look at the RV in advance, we arrived there one Saturday morning in a packed station wagon not quite knowing what we would find. (They had a locked area in which we were to leave our car.)

If at all possible, visit several different rental firms to inspect their vehicles and determine just how much space and amenities you want to pay for. For instance, our RV was rated as sleeping six, but as least one of the beds was not big enough for two adults.

The Tioga turned out to have a roomy body mounted behing and over a heavy-duty Chevrolet van cab and chassis. There was plenty of storage space for all our luggage and gear. Since it qualified as a truck-and handled like one on the highway- it used leaded regular gasoline, a fact that provided quite a saving, since it got only about eight miles to the gallon. (Filling a 50-gallon gas tank is a familiar experience for RV users!)

Fredericksburg RV Rentals had required a $250 deposit to hold the rental, considerably more than other places we checked. On the other hand, their priced were a bit less, and there were no extras. Cruise America in Clinton, Md., for instance, charges either $4 a day or $2 per hour for use of the gasoline-powered electric generator on board, even though you still pay for the gas. Other firms charge fro certain insurance coverage.

But we were charged nothing extra for insurance-an important point to inquire about, since these are very expensive vehicles. Uncertainty about insurance was the major reason we chose not to pursue any of the newspaper classified ads offering personal RVs for rent.

In this case, there was no deductible on normal automobile-type collision damage costs. Instead, it was explained, we were responsible for the first $250 worth of damage that was not normal collision damage, such as ripping off a top corner of the high RV on a low-hanging limb. (This was brought home later on Jekyll Island, Ga., we ended up driving briefly on the wrong side of the road to avoid some live oak trees draped with Spanish moss.)

On our rental form, the $250 deposit became a security deposit. The remainder of the the estimated charge. $70 a day for eight days plus 12 cents a mile for an expected 1,500 miles, came to an additional $740 payable before we left. They took Visa and Mastercharge.

Also before we left, the RV's various features were explained to us. The explanation was thorough, but we found later that we had not absorbed it all. We wished we had listened more carefully, and asked more questions. A manual found in a cupboard was not much help, and it took us most of the week to fully understand all the intricacies of the RV's water, gas and electrical systems.

Hooking up the RV when we stopped at night provided not to be difficult, and in later days became routine. The RV's water pump had to be switched off and a garden hose run from a fitting on the side of the RV to an ordinary outdoor faucet. A heavy electrical cord kept in an exterior compartment had to be plugged into a waiting receptacle. And a large flexible pipe stowed in the hollow rear pumber had to be attached to the outlet at the rear of the RV and inserted into a ground-level drain. When the two blade-type valves were pulled open, waste water and sewage accumulated during the day in two separate on-board holdings surged into the the drain. (Often RV camping areas do not have drains at each site but only a central "dump station" at which the holding tanks are emptied.)

In addition to the three hook-ups, we had to light the pilot light on the water heater. It was readily accessible in another exterior compartment, but it was hard to light and on a couple of occasions did not stay lit overnight. And the refrigerator, which ran on propane or electricity, had to be switched to the latter. The RV also had a gasoline-powered electrical generator, which would run the air conditioner, the refrigerator and the electrical outlets, but we used it only briefly since we had an electrical hookup every night.

as with any home, it was useful to have a toolbox. For instance, one of the handles on the holding-tank blade valves slipped off and pilers were needed to pull it open. A garden hose was provided in the RV for water hookups, but we carried a longer one just in casw. We also took and used a very long, heavy extension cord for hooking up on one occasion.

The rental cost for the RV and the 1,523 miles driven turned out to be $769.68, including tax and $4.50 for propane. Two weeks after returning the vehicle, we received-as promised-a check for the balance of our deposit. Our other major cost was for 186 gallons of gas.RENTAL FIRMS: You can rent an RV at the following firms in the Washington area: Bay RV Service Center, 10 Old

Solomons Isle Road, Annapolis,261-8016 (Washington telephonenumber). Cruise America, 8201-A SchultzRoad, Clinton, Md. 856-3530. Family Trails RV Rentals, 11732Annapolis Road, Glendale Md.,464-2711 (Washington telephonenumber). Fredericksburg RV Rentals, U.S. Rte.1, three miles north of Fredericksburg, Va, 385-1323 (Fairfax telephone number).

P & G RV Rentals, Rtes. 15 and 55, Haymarket, Va., 385-4482 (Fairfaxtelephone number). TRavel-Ho, 17 Fairfax Court, Sterling,Va., 450-6435. U-Haul Co., Four locations: 5005Duke St., Alexandria, 370-4412; 1107Broad St., Falls Church, 534-2345;12025 Parklawn Drive, Rockville,231-9469; 6889 New Hampshire Ave.,Takoma Park, 270-6400.