the one thing the OP needs to understand
is that the Magnum inverter/charge combo
has absolutely NOTHING to do with the alternator
no matter what the tech on the phone told him
he has a break down in the chassis to house charge circuits
loose connection, bad isolator charge control, or one side of a dual output alternator that is out, blown fuse in the alternator to house batteries circuit
yes.. if the inverter is on powering a heavy load, it will suck on the batteries
and INVERT mode is normal IF there is no 120vac coming in
he didn't say but i wonder if it is an ALL ELECTRIC coach
{ if so he has to have the inverter on or generator on when not plugged in }
and blew out something on the chassis charge input side