i'm just a bit confused as to what you are asking? you say to try to run the generator? does it run? you mention automatic transfer switch. do you think it is not switching when you run the generator? are you trying to run the heater when the generator is on or off? if the generator is running, and if the transfer switch switched to generator power, there should be power going to the converter / charger which should allow the heater to run. if this is not the case then the heater is powered solely by the batteries and if they were low that could affect the heater operation. can you measure the voltage on the batteries with no shore power or generator running? this should provide a clue as to how charged they are. then start the generator and remeasure the battery voltage. if it increases the transfer switch is providing generator power to the converter / charger which is trying to recharge the batteries.
2015 cardinal model 3825fl
2015 dodge ram 3500 dually