Keystone Hideout 26RLS Power issue- trouble shooting help needed


New Member
Long story, short version:
A family friend stayed a couple nights in our RV, he said he switched on the fan on the thermostat but he mistakenly turned on the AC and it was pulled into 100FT 12g extension cord to maintain the batteries.
Now when on battery power the furnace works fine like it should, but when you plug into AC power no matter if you set the thermostat to Fan, AC or Furnace the overhead AC unit starts. If you are on battery power running the furnace at camp it runs fine, plug in a generator and the AC starts without changing the thermostat from furnace to AC and over loads my little Honda gen i used to charge the batteries. If i switch the thermostat to off, then the Honda can charge the batteries.
It has the WFCO WF-8900 power converter.
I understand electricity and am familiar with using a fluke and troubleshooting house hold appliance aslo trouble shooting engines etc but have no exp in RV trouble shooting, my first RV.
Im looking for ideas or pointer on what I should look at, thank you for any help