RV water heaters are generally designed to mount in a flat exterior wall, and the solution is mould a surrounding adapter (or transition piece) in fiberglass, which is visible in photos of trailers such as my Boler
1700 (see the same setup on a different variation of B1700 in an Album photo
). With the water heater, it's a large square heater door, so the whole thing is quite a wart on the curve of the trailer, but for a typical furnace
vent it could be quite small and blend in much better.