Actually, we replaced the Coleman Mach with a Houghton AC/heatpump this fall
and are very happy with it. It works down into the 30s and is quieter and more consistent (so Brenda says) than the furnace.
We did have to rely on the furnace at NC Alumalina in October for the first night as the 30a breaker at site was on the fritz (and so happy we had the furnace for back up).
But with the Houghton, we can save our propane.
(Probably won't last as long as Coleman, which is still operational, as nothing new ever seems to.)
Probably always a good idea to have a propane furnace as a back up and given the danger of heat exchanger leaks from pinhole rust, best to ditch the original if over 20 years old (IMO).