There are a number or RV's with tubs so that's not really any kind of an issue other than trying to determine is there is room for such a thing. You might have do some serious research to find a tub that will fit the space but it's certainly doable. If all the room you currently have is the space the shower occupies then finding the additional space might be your biggest challenge. The toilet can be moved (make a plumbing plan first!) and a vanity can be moved or reduced in size.
It's just going to take a bit of planning but if that's what you want, and you have the budget for it, that's what you should get.
