[exjay]

That tub should have a _] shape that goes behind the wall to drain water back into the wall which is why there is a gap. That little lip that runs along the tub outter edge isnt much i know but if that lip is level (along with your camper front/back and side to side) you technically shouldnt have a drain issue getting water back into the tub i would think if installed correctly. If your base is sealed up like you say, make 100% no water can get behind the wall such as resealing the faucet fixture, shower head mount and any screw holes etc in the wall panels. Maybe if you wanted to DIY without tearing out the tub, build up that tub lip with silicone or glue a rubber sweep/squeegee etc along the length to help keep water in?.. good luck!