Re: Frame rails as drain traps?
The spa pipe i also used for some applications.. when you glue spa pipe to a regular fitting, make sure you let it sit for at least over night, if you are using the normal cleaner and glue. I have found that the drying rate takes far longer than cpvc pipe. if you can twist them out after the next day... which i found sometimes i could, you can always put a couple of short sheet metal screws in to hold the joints together... course, epoxy works great as well.
the spa pipe is great for slight bends, etc. i used it to connect the toilet to the black water tank as it will deflect a bit if the tank shifts some whilst driving. The other thing that i used in a number of places in the drain system was the rubber connectors for 3 inch, and the reducing ones, etc. After having 4 other busses, i wanted to have some flexibility between each of the holding tanks - 3 grey ones, and the above the floor drain system.
just fwiw, i trust the spa pipe... it is thick and never will wear out. I am not so sure about the clear pipe, which i have some, but havent used.
Jesus Christ... Conversion in progress.