So with no house charging from alternator, the engine battery showed low voltage as though no alternator, but new alternator same thing. You might have had a big house draw that should have been taken by the house batts that was taken by the engine batt, lowering the voltage.
I saw that with a loaded inverter clamped on my truck engine batt where the alternator could not keep the battery up and its voltage fell. (Not a MH situation, but still...)
First thing is to isolate the house from the engine entirely and make sure the alternator can run the engine batt and straight truck type loads ok.
Then see what is going wrong when you link in the house side.
