First, realize that BOTH batteries are in disconnect mode, zero volts present anywhere except to the momentary switches above the door.
Turning key does nothing, nor does any other action.
If coach batteries are below the voltage required to pull the start relay in, it will not do so.
The power for the battery controls is sourced from the coach batteries which are too low to work in this case.
Thus the starting battery is not connected, period.
Pushing the boost switch connects both battery systems together which will allow the relays to work and connect the battery, problem is the boost switch is momentary and mounted on the dash, the battery control switches are mounted above the door, requires helper to push one or the other.
Tony & Lori
1989 Country Coach Savannah SE