Frame rails as drain traps?

[bus-bro]

I'd route the drains under the rails into the low side of the tank. Plumb in a clean out. That going up & over the rail just feels like bad ju-ju.

 

[somewhereinusa]

I found this on a search for trap distance

There is usually a maximum allowable vertical drop from the fixture outlet to the water seal. Why you ask? Because the further the liquid waste drops the greater the velocity of the water. The greater the velocity of the water rushing through the fixture drain the more likely the drain will siphon the trap seal. The vertical drop for sinks, lavatories, showers and bathtubs is between 18” and 24” but not more than 24”. The shorter the vertical distance the more efficient the trap functions.

I bought these for my conversion, since it doesn't hold water, distance shouldn't be a problem, and you don't have to worry about freezing in cold weather.
http://www.hepvo.com/HepvoUSTechGuide.pdf
Installation manual here, has good explanations
http://www.hepvo.com/HepvoUSTechGuide.pdf
From the RV Doctor
http://www.rvdoctor.com/2011/03/curious-about-plumbing-beneath-bathroom.html

My understanding is that you don't need an external vent with this type of trap.

 

[chev49]

i think all you gotta worry about is a method of clean out / disassembly if you got something valuable or its full of combs or rocks. Mine bus is the same way, but i left the tanks down just a bit.

 

[bus-bro]

I'd slope down to the tank. A slight rise there might clog with gunk.

 

[chev49]

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.

 

[chev49]

i think the reinforced clear plastic pipe is good. i thought you were referring to the thinner clear stuff...