can you measure the voltage on the two large lugs with engine running ?
do you have access, or trace the wires back
if that is the 'charge control' solenoid (and not working) one side will have alternator voltage and the other side will have house battery voltage
if both sides have the same voltage low 12.1, device might be working and problem is elsewhere
if one side is high aka alternator and one side is low, then that is the jump start / charge control and it is NOT working
so check the voltage across the two small wires, it may not be getting the command turn on power
the BCC (battery control center) are a know problem, that does fail
usually the control board or one of the fuses on the board,
the house battery system disconnect
disconnects the batteries from the RV systems, meaning NO power to the RV lights etc. NOT to be confused with the charge control/jump start relays
