It depends on how the condition of the gel coat/fiberglass. I charge $16 for foot typically if it is a basic bass boat or center console. Vacuuming a cabin, dealing with odors, and brightwork/teak is extra of course. Not a lot of RV's in my area. Maybe someone else will chime in.
Other detailers in my area charge $10-$13/ft but boats are hard work. With a 20' foot, I'm looking at $320 with 6 hrs of work. It's good pay but I'd almost want to just do 2 cars instead. Now if I have some downtime coming up, I can always fit it in. I find that boats are not like cars in the sense that clients want it done asap. Its more like you have a 2 week window to fit it in soon.
The real winner is doing their boat and then selling them on their cars too. The boating community is a good little niche to get into because they tend to have more disposable income and everybody knows everybody. If you do great work, they'll definitely pass your name along. 1 satisfied car client tells a friend. 1 satisfied boat-owning client tells every boat he ties off too and every random stranger who tells him he has a nice looking boat.
Good luck with the details.