It's certainly not typical, but people do do it. The older an RV gets, the more in need of a facelift it gets. The tricky part is you can't just waltz down to your local Menards and buy 2x4s and drywall and start nailing stuff together. Everything in an RV is oddly shaped and sized, and there are weird trim pieces and vinyl bits everywhere, and you're always concerned about overall weight.

So, no clawfoot cast iron tub in the bathroom for me!