bertnovak

You are  free to agree or not but please measure the voltage being supplied to the solenoid and the starter when trying to start the engine. A bad ground to the engine block will show a a very low voltage to the starter when trying to start. If you see 24 v at the solenoid or starter you have a wiring issue probably at the battery. I have seen 12v batteries connected in series for the starter (24v) not uncommon for heavy equipment but not in motor homes. Are you SHURE the Starting batteries are not connected in series when they were replaced?

I believe you have wiring problem at the small relay connected to the ignition switch or in the battery box. Probably supplying 24V to the starter(Large) solenoid. Burns out the coil of the solenoid. The starter windings can take the overload. 

When starting and plugged into shore power the charger is still  putting energy into the system.