I believe the HTR button is your water heater not the air heater. And then of course the pump is your water pump. Regarding the air heater, it should all be controlled at the thermastat, just set the temp you want, be sure your propane is turned on at the tank, if you have two tanks, turn one on and leave the other turned off until you need it.
Looks like that is a digital thermastat only so if there are no button on the therm for heat or AC then it should be in the menu somewhere on the thermastat. That is all you need to do.
Also just a little tip, when it gets cold i always cover up my inside vent to the roof AC because heat rises and we dont want to heat anything but the inside of the RV. I use an old windshield sun shade from my truck. I cut it to fit over the AC roof vent (inside the RV) and then used velcro type stuff so i can remove it when needed. Just remember NEVER NEVER NEVER turn the AC on with that cover on. I also have a note on mine in big letters TURN AC OFF FIRST lol