Not hard getting the leads REVERSED as the connectors on the battery terminals are the same type.
If they were reversed you may have blown two FUSES in the 12VDC Distribution Panel labeled 'REVERSE POLARITY' and then another in-line fuse close to the battery bank.
Don't feel bad blowing the fuses as that is why they are in there - to protect from us guys hooking them up in reverse. No Harm DONE... Just blown fuses... You are not the only one that has done this haha...
Be sure to correct the wiring before you replace the fuses otherwise they will just blow again...
I always look for the word 'NEG' or the symbol '-' Stamped into the outside 6V battery case and make sure this is he battery terminal that goes to FRAME GROUND...
The normal trailer wiring colors will be BLACK for positive +12VDC cables and WHITE for negative -12VDC cables. They may white cables and the ones with color stripes will be positive leads...
Sometimes however they will be using the AUTOMOTIVE Battery Colors which is RED for positive +12VDC and BLACK for negative -12VDC cables.
Roy Ken