here are many tanks available for both water and sewage. Unfortunately, finding one that gets the maximum storage in the space and shape available in your coach may be difficult. About the only way to get a tank that just fits is to make it yourself. The tanks I built use fiberglass on plywood techniques. The plywood is for structural strength and shape and the fiberglass is for waterproofing and additional strength.

There are other ways to make tanks such as ABS plastic sheets or stainless steel. If you are familiar with these materials and have the necessary tools, you could use one of them. Otherwise, fiberglass on plywood is probably best. Most people can work with plywood and the fiberglass part is easy to learn and takes only a few inexpensive tools (scissors, paint brushes, paint rollers). Making your own tanks will save some money, but not enough to justify the work. If you can find a commercial tank that fills your needs, use it. If you want to get the most water in the available space, you will probably have to make the tank yourself.

The coach designer must make a decision on how much water and sewage capacity to carry and then try to fit it in. My experience suggests that two or three gallons per person per day are needed without showers and four or five with. If you have a marine toilet that uses water with each flush, add another gallon or two. How you use your coach will be a prime factor. Someone staying in RV parks needs far less capacity than someone who boondocks. Another approach is to follow the general practice used by others with the same size unit. For bus conversions, 100 gallons is typical.

The water and sewage tanks do not need to be the same size. It is easier to find water than a dump station, and the mess you have is far less when you run out of water than when the sewage overflows. Another consideration is the weight of the water. At 8.3 pounds per U.S. gallon, a full one hundred gallon tank will weigh 830 pounds plus the weight of the tank.

To find the capacity of a proposed tank, multiply the inside length times the width times the height to get the total cubic inches inside the tank. Divide this by 231 to find how many gallons the tank will hold.

The code that covers tanks and the plumbing system connected to them is ANSI A119.2 "Recreational Vehicles". For tanks that hold human waste (black water), it specifies that all openings for liquid or waste coming into the tank must be on the top. This restriction does not apply to gray water tanks. If you only have one tank, it must be made to the standards for a black water tank.

The material generally called fiberglass is more properly called Fiberglass Reinforced Plastic (FRP). The fiberglass reinforcement comes as woven cloth or mat, while the plastic comes as a liquid. When another chemical, called a hardener, is added to the liquid plastic, the plastic slowly turns to a hard solid (cures). For this application, the plastic should be a polyester formulated for laminating fiberglass to wood. The materials are applied so that the end product is a hard waterproof layer of plastic heavily reinforced with thin glass fibers.

There are two basic ways to make a tank using the fiberglass on plywood technique. The first is to make the tank out of plywood, assembling the bottom and sides and then fiberglassing the inside and the inside of the lid. With this method, a flange can be left around the tank to secure the tank to the coach. You can also make the lid removable. The second method is to cut the plywood pieces and fiberglass each one first. The tank is then assembled using an epoxy cement and a few layers of fiberglass over each edge to hold the joints together. This technique is easier, but a mounting flange and a removable top are not practical. A lot more work is needed with the assemble-first technique to get both the inside and the outside corners rounded so the fiberglass will lay properly. It is also much more difficult to lay the fiberglass on the vertical sides.

Locating the openings for the fill, the drain, and the vent is part of the designing process. With either technique, the holes are cut before the tank is assembled or fiberglassed, but the fittings are not installed until just before final assembly (putting the lid on). ABS, PVC, or CPVC fittings are used to make the connections. A fitting is cemented onto a short piece of pipe placed through a hole in the tank. Another fitting is placed on the pipe where it comes out so that both fittings are tight against the tank and cemented to the pipe. Epoxy putty is used to bond/seal the fittings to the fiberglass.

Baffles in the tank are needed to prevent damaging waves from forming when the coach stops abruptly. I made these from half-cylinders cut from plastic drain pipe. They were bonded to the under side of the lid and went down near the bottom, but enough room was left so that waste could flow out unobstructed.

The last Time I made tanks was 25 years ago. However, I am planning two tanks for my present project and have purchased most of the materials. The black tank will be 29 by 27.5 by 20.5 inches for 62.5 gallons. The gray tank will be 43.5 by 27.5 by 20.5 for 95 gallons. My known and estimated costs are: Cloth $67.43, Mat $32.50, Polyester $114.95, Plywood $60.00, Fittings $20.00, and Epoxy Putty $49.95 for a total of $344.83. This is a little over $2 per gallon. Because I am converting a transit and need to fit the tanks in specific places, I need tanks that are exactly the size I want.

The fiberglass on wood technique can be used in other places. I used it to make the shower pan for our first coach as there were no commercial pans in the size we needed. I have also used it to protect the deck of a utility trailer, but the primary use is to protect and waterproof small boats. The project that got me going again on fiberglassing was making a front cap for our new coach. Because the per unit price drops as you buy more, I bought the materials for both projects at the same time. One five gallon can of polyester costs less than the four one gallon cans I needed for the tanks, and is enough for both projects.

The retail price on two tanks of about the same capacity from Discount RV would be $231.95 for the black and $273.95 for the gray, totaling $505.90. I get my fiberglass and polyester from Fibre Glast Developments Corporation

(Editor's Note: Prices quoted were current U.S. dollars as at the original authoring date. They are shown here as a comparative study only and not meant to convey actual costs as you read them today).