Unplug the converter/charger from the outlet on the floor. Tape the positive lead from the battery so it does not touch anything.
Plug the trailer into grid power. Turn all the 120 volt circuit breakers fully off, then back on to re set them in case any have turned off for some reason. Then your 120 volt outlets should all work, and maybe the AC unit if you have one. If there is a refrigerator, turn it on to "electric" (assuming it is a gas/electric one, of the original design) and in 4 to 8 hours it should be cold, at least in the freezer compartment. It will make no sound. It runs with a heater, not a compressor. If it is a newer refrigerator, it might require 12 volts to run, so the above test would not work without 12 volt
Plug your converter/charger in and you should have lights and fans and pump operation, even without a battery.
That will get you started at least.