I think I may have misunderstood your post.
Is this in one of the storage compartments? If so, itprobably isfor some sort of plumbing but I am pretty sure there is no trap for the toilet. It just drops into the tank. The trap in my 5er for the tub is visible in the storage area because the "ceiling" of the storagecompartmentis the subfloor for the bathroom above it so the trap just "hangs" in the storage compartment. I do have a wooden panel that is screwed or bolted (can't remember which) to the "ceiling" in the storage compartment that is where the outside shower and toilet supply lines run. When I first read your post I thought you where talking about underneath the camper. My rig had a panel made of coroplast that had 4 bolts in it that was located where the slide out motor was.
Lonnie