Motorhomes and home GFI circuits do not play well together. The reason is the different way that grounds are handled in the motorhome. You are well protected in the motorhome with your own GFI circuits. Plugging into an external GFI can be an adventure - it may work fine and then trip, for no apparent reason, in the middle of the night. For that reason, I recommend that people who store their motorhomes at their house, install a 50 amp receptacle and breaker. For 30 amp coaches, use an adapter or dog bone.
As to setting your Magnum Panel to limit the amp usage for the charger, you should only have to do that at the Magnum panel. If you have a load sharing system, it will sense the incoming power as a 30 amp circuit or less and pretty much take care of things from that end. If you are plugged into a household 15 amp circuit, you can't run more than the charger and the refrigerator anyway.
Roger & Mary
2017 Winnebago Navion 24V
2014 Tiffin Phaeton 36GH (Sold)