Instead of messing around with a 25+ yr old furnace, perhaps try some RV salvage yards or RV SURPLUS PARTS STORES?
COLAWRVSALVAGE might be a good place to start..
Found a NT30 furnace (not sure if new or scratch and dent) for $495
RVSURPLUSPARTSRVPARTSNATION is where I scored a Scratch and dent Atwood 30K furnace quite a few years ago for $300.
Atwood would not be a direct fit but if you can score a good price it might be worth the hassle.. Was worth it to me since the Suburban furnace I had was 25+ yrs old at the time with a bad control board and the heat exchanger was extremely rusty..
HERE is a websearch for Surplus RV furnace.. Lots of places to look.
