Common problem back in that timeframe - we had it but just added an external vent to the door with the louvers facing the correct way and re-positioned the circuit board.
Your furnace should be pretty much removable from the outside but if you need to get to the back it looks like your access will be from underneath the RV as it's below interior floor level.
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad