Electric Problem Need Help

[Trapper57]

I have a 2021 Montana 3121 with Solar Fex 400. This has happened to me twice the vary same way. We set up and run on solar and all is going well, at end of second day worried i will not get through the night on the battery so start up my portable generator and plug it in and run for a few hours then turn off generator for the the night. Later in the night I run out of battery and fridge, lights everything stops. I get up start the generator in the middle of night and I get no power in the Camper. I have to wait till in the morning and the sun comes up and charges the battery before I can move my slides back in and run jacks to hook back up to truck. I finally get hooked up and pull back home and plug into electric cord in garage and everything then runs fine. I have never had a problem with camper switching between battery, solar and elecitric except when I use the portable generator, and the generator does work. WHAT IS THE PROBLEM AND HOW DO I FIX IT ?