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.