Our new Sunseeker has a single thermostat for both heating and cooling, however, the two functions are in completely separate units. The ducted propane heater is under the refer, and delivers hot air at the floor level. The Dometic Roof top AC is ducted throughout the ceiling. There are no controls on the intake panel on the AC unit.
If you have a single thermostat for both, it's possible your gas is not turned on, or there's a problem with the power or wiring to the heater.
Older AC units may have an optional heat strip but it's pretty low wattage. It may help to keep a unit warm in moderately cool temps, but it is not sufficient to warm a cold unit, nor is is sufficient to keep up with cold & windy environments.