The genset is an ONAN 4000 or 4100 (can't remember) whisper quiet model. I have not checked the circuit feeding the charger/controller; it is hardwired. If I understand you correctly I can upgrade to a 75 amp charger only if it is a 20 amp circuit.
My reason for wanting to upgrade the charger is two-fold. One is to reduce the run time while charging. The other is to allow me to leave the unit plugged in all winter. I keep the batteries up so that I can shovel a path to the door and run the genny once a month. The batteries should last much longer with a smart charger than the stock unit won't they?