I think PT is saying the Magnum reduces charging at 122v input (some kind of thermal ? protection for the Magnum )
turning on the water heater (using genny power not inverter) reduces the yammy output line voltage to under 120v or less and the magnum then turns on and charges the batteries
he just recently got an auto transformer for Boosting low voltage shore power conditions
but it sounds like he needs to 'BUCK Down' the generator output, because of the finicky Magnum
it Is an unusual situation
Don
don't you have another converter you can plug in in this situation
39amps is no where near the max of the magnum
IIRC you do have something else that will supplement that 10 amps and get back to near 40amps charge