[Richard]

Bill, the manual disconnect switch should take the house batts out of the starting situation but if you want to make absolutely sure disconnect the AGM positive and negative leads. Tape off the positive, so you don't create a short. Turn off your charger/inverter first. That will absolutely disconnect the chassis and house batts. I don't know what charger inverter is in that coach, but keep in mind, that many of them automatically default to charge when they are plugged into shore, or the gennie starts. So you may have to turn it off again.

The other thing that could have happened is the AGMs froze. Batteries can freeze if they are discharged. Either way they are toast.

Phone number is 817 223 2056. Glad to chat if you needed.

Good idea on the heater.

It is possible the fuel is gelled, the heater will help with that.

The two strokes do not like to crank cold. The heater will help with that.