I don't think Thor designs the coaches to charge the engine battery from the coach battery. However, the coach battery should charge through the engine alternator when it is running.
There should be an emergency start switch that allows you to start the engine from the coach battery though.
I think they opted for charging the coach battery via the alternator rather than charging the engine battery via the coach battery, as they provided an emergency engine start off the coach battery.
I live in Michigan, so I might experience similar winters as you do. I start my coach in the winter time about once a month, and that is sufficient to keep the engine battery charged.
This might not be sufficient for some of the Class A coaches, but the E series Class C coaches do not seem to have as many "parasitic loads" on the engine battery (stereo, etc.).
I also start the genny at the same time (under a 50% load), as if you don't periodically run the genny, you may have problems down the road.
The only thing that works on a RV is the owner...