The Fan-hi and Fan-lo wires should go to the A/C unit
Only the white wire controls the heater.
It's not making sense to me, my guess is that the thermostat electronics might be hosed up
2008 F350SD V10 with an 2012 Arctic Fox 29-5E
When someone tells you to buy the same rig they own, listen, they might be right. When they tell you to buy a different rig then they own, really pay attention, they probably know something you don't.