Old trailers had waste (black water) tanks under the toilet, but no wash water (gray water) tanks. The wash water just dumped straight on the ground. Today, it's illegal to do that in most states, and very few campgrounds would put up with you if you tried it. New trailers all have separate gray tanks.
Below you can see the frame modification we made to accomodate a big gray tank under the floor. It will sit between the wheels. The space is 50" X 35" X 3 3/4". That equals 6532 cubic inches, or room for a 28 gallon gray tank. For comparison, new Airstream trailers up through 23-ft long have 21 gallon gray tanks and black tanks of 18 to 21 gallons. They have fresh water tanks as big as 30 gallons.
Frame Rung Moved Forward