Since the boost switch worked, the Big Boy is probably OK. The voltage to actuate the Big Boy would come from either an Isolator Relay Delay (IRD) or a Bi-directional Isolator Relay Delay (BIRD). If you have an IRD, being plugged in will not result in charging the chassis batteries. If the chassis batteries not being charged is something new, then there is a BIRD somewhere that may be acting up.
The way it all works is that the IRD or BIRD must see at least 13.3V from either the alternator or the coach converter for at least a minute before it will send the connect signal to the Big Boy.
To test it all, be sure there is more than 13.3V at the chassis batteries for more than a minute.
If there is, measure the voltage between either of the smaller terminals on the Big Boy and ground. If there isn't at least 13.3V on the small terminals, then it is either IRD or BIRD which ever you have.
If there is 13.3V on the small terminals to ground, then the Big Boy should be connected and the voltage on each side of the larger terminals will be at least 13.3V relative to ground. If the voltage doesn't match, then the Big Boy isn't connecting and is at fault.
Here is a link to a BIRD wiring diagram:
Here is a link to an IRD wiring diagram: