Travel trailers in most cases do not have battery switch.
The reverse polarity fuses vary per type of converter. A stand alone style have the fuses in the DC end of converter. Converters that are integral to the power distribution panel are located in the 12 volt DC power distribution panel. Next to the 120 volt circuit breakers is a small plate held on by thumb screw. Fuses are behind plate.
Some AC units use 12 volts DC for control, so it is possible that is why it doesn't work.
Your power problem maybe that the batteries are not wired correctly. Two six volt batteries should be wired in series. The positive of one battery should be connected to negative of the other battery. The positive of second battery goes to DC panel through a fuse or 12 volt circuit breaker. The negative of the second battery should be connected to frame of trailer.
I would stay away from using 12 volt DC for the refer. Draws a lot of power at 12 volts.
Make of Tow vehicle GM products, you need to look in owner's manual on ho to connect charge circuit to trailer connector.