I'm pretty sure there is room for a shallow cabinet in the wheel well space under the closet in our 13' Trillium
. I've seen pics of heaters installed there. Pull the bottom shelf of the closet out and you should be able see what space is available. I think you could put a compartment on the door so you don't have to make a cabinet just hinge a door on an opening you cut in the fiberglass. You could also have it hinged at the bottom and tilt out like a tilt out by a sink. That would eliminate a door swinging into a congested area if you are in the galley.