If you have WHITE and BLACK wiring then the WHITE WIRE is NEGATIVE -12VDC and must go to NEGATIVE (FRAME GROUND) Battery Terminal. The BLACK is POSITIVE +12VDC and must go the POSITIVE BATTERY TERMINAL.
In the AUTOMOTIVE WORLD it is different where RED IS POSITIVE and BLACK is frame ground. Makes for big confusion between the two worlds or RV and AUTOMOTIVE where BATTERIES are being used.
You most likely have blown some fuses... There is a FUSE or CIRCUIT BREAKER real close to the battery and also TWO HIGH CURRENT RATING FUSES located in the 12VDC Distribution Panel labeled REVERSE POLARITY...
Be sure to find the markings on the BATTERY CASE that says NEG or the SYMBOL - and make sure this is the WHITE WIRE CONNECTION. I would also hand trace the WHITE CABLE and make sure this goes to the trailer FRAME GROUND near the battery. Someone may have rewired it before you.
Roy Ken