My concern with the bathtub would be if the mattress isn't completely tight against the sides of the tub it can become a suffocation hazard. (If the baby were to roll into the gap with his/her face up against the mattress.) We're tent campers, so I've never actually seen a bathtub in a camper.
I know you said no co sleeping and I don't know what about it that makes you uncomfortable, but what about something like this? (Sorry no clicky, on a Mac)
It'd keep LO safe in a bed and stop anyone from rolling. (If that's your concern.)