Look for blown fuses in the power distribution panel labeled REVERSE POLARITY. The older trailers usually had these two fuses off-set from the regular Dc fuses. My setup also has an IN-LINE fuse located real close to the battery install that would blow as well...
You can see the two large current fuses off-set to themself in this photo..

I dabbed each of my POSITIVE Dc cables with RED FINGERNAIL polish to keep me straight on which battery cables goes to the POSITIVE TERMINALS... Very easy to mess up with the RV trailer batteries that uses the same size terminal for both NEG and POS connections. The automotive industry uses a larger connector for the POSITIVE side...
Should be no harm done as fuses protect everything when this happens...
Roy Ken