Hi Keith,
What kind of batteries do you have? Are they AGM, or conventional "wet cell". The Magnum is a 3 stage charger when plugged into shore power or on generator. What does the remote panel say, are there any fault lights on? If the voltage is above 15 volts, either the charger has a bad voltage regulator inside the charger or it is seeing an issue with the batteries. Dirty cables wil make the charger think that the batteries need charging and cause it to come out of "float" (maintenance charge) and go into "bulk" (maximum charge). Is there a sulfer smell or are the house batteries hot to the touch? How old are the house batteries?
Regards Fred