Pretty sure it's AC, DC, LP and Auto (not Aux).
Yes, it should run on house batteries - but you want it to run on LP when not plugged in. So, put it on LP or Auto. When on Auto and you are not plugged in or running a generator it should automatically choose LP.
Are you positive it has "DC" as a fridge option. It may, I don't know about an 18 year old Minnie Winnie specifically.
But if it has DC that will use up your battery(s) very quickly. So, choose AC when plugged in and LP when not. Or use Auto when you get the hang of it's operation better.
2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad