Switch at floor level near the entry door is probably a 12V DC light switch for the light behind the entry steps.
Most interior lighting is 12V DC.
If you're plugged into shore power, the Power Converter should be supplying power to your lights, even if the battery is dead.
If the battery was recently changed or reinstalled, the fuses on the Power Converter might have been blown. That would cause your battery to run down, and you wouldn't get any 12V output from the Power Converter.
If that's the scenario, plug the trailer into your truck and start the truck. If the lights come back on, you'll need to investigate the Power Converter which is located behind the rear wall of the pass through basement storage.
Don't leave the trailer plugged into the truck - you'll run the truck batteries down.
If you have an automotive battery charger, you can plug it in and connect it to your battery terminals as a workaround.
Our
12V Block Diagram and Diagnostic Guide may be of help.