I am a TCer who just happened to decide to look at Class B forum (no new posts in my favs....LOL).
Not sure at all if this helps, but pop-up people use laundry sinks. You can cut them and mod them pretty easy and they come in various sizes. very strong and can be used as a storage tub when not in use. Years ago a friend had a DIY camper van and used one. His had molded plastic legs (which he cut down) so his "plumbing" had room to come out from the bottom. he ran a hose into an outside tote and it was a gravity drain thing... but I have seen the flat bottomed ones used too.