[Brit Road Trip]

Hello folks, hoping to receive some troubleshooting assistance with my inverter. I have a 2021 3121RL 5th wheel. I have complete power when connected to shore power. When I disconnect there is no power coming in to run the Fridge, water pump, interior lights. Before my road trip, I purposely ran the fridge and lights to test how long the battery power would last. I stopped after 4 hours feeling satisfied. During my 2nd leg of the trip I noticed the CSW inverter was not displaying any voltage when we stopped in a rest area. When I connected to shore power later that night at the campground, the display had to be turned on and showing 12.9 volts. I have checked the batteries before and after the disconnect and at the input of the inverter, showing 12.9 -13.0. Why is there no power inside to run the fridge, pump and lights ? thank you, hoping for a simple fix to avoid taking it in for the repair.