Consider using the smaller furnace and adding a small catalytic, or
even going pure catalytic.
We're taking the latter route on the '73 Safari (23') we're reworking:
NO electric draw, less weight, and we gain space by having removed the
old forced-air furnace and its duct work. We don't plan on as much cold
weather camping as you apparently do, and as the project is far from
complete we can't even tell you how well it's worked out. Others on the
list can tell you more about their catalytic experiences, but, based
significantly on some of their good reports, we're pretty confidant of
this solution.
Rob