It takes a LOT of current to make that happen.
The most common cause is shorted contacts on the starter solenoid, keeping the starter running (or trying to) a lot longer than it should.
The second most likely cause is a short inside the battery itself.
The charging equipment in an RV is generally not capable of pumping in that much current, unless it has had a massive failure internally.
EVERY RV OWNER NEEDS A VOLTMETER.
Then.....turn that switch back on and see what happens.
AFTER you get a voltmeter so you can monitor what is happening.
IF your chassis battery was cooked by the charging equipment in the "house", then your house batteries should be toast too.