The NT series has been around forever, and is a decent furnace. For your use, occasional and in mildly cool weather, the 16,000 btuh output of an NT 16 would most likely do fine.
When I had my 310 motorhome it needed a new furnace. I replaced the original NT 30 with an NT 24 (at that time available) and it worked just fine. I camped in it as low as 16 F, and it was a 31' long motorhome with lots of glass and surface area.
Because of the physical size differences you may have to modify or build a platform to mount a smaller furnace where the larger one was. In general, the NT Suburban air inlet and exhaust holes are located in the same configuration no matter what the capacity, but the furnace box itself could be different.