Are you trying to transfer the load while hooked up to shore power? Or just getting into the RV and cranking up the genny?
If Cummins is saying the genny is good, meaning its putting out power and works above freezing, then I would suggest your transfer switch is sticking.
Have had that happen to us on occasion. Although it was not temperature dependent.
Carried a rubber mallet expressly for the purpose of whacking the switch when it was reluctant to move. Might give that a try.
Or move to warmer climates......