The best method I have found is to find the word "NEG" or the symbol "-" on the 12VDC battery case. That terminal should go to the trailer FRAME GROUND connection which is close to the battery install... There are too many different wire color schemes to say use the RED one or the BLACK one or the WHITE one... Follow the negative terminal wire to frame ground works out best for me...
If you get them reversed just for a second it normally instantly blows three fuses. An IN-LINE fuse close the battery and two fuses in the 12VDC POwer Distribution Panel labeled "REVERSE POLARITY"
Once you get the batteries hooked up correctly I would identify the wiring with something. Some folks will mark the POSITIVE LEADS with a big DAB of RED FINGERNAIL polish. Others like to take pictures of the wiring with their CELL PHONE cameras.
Happens to all of us one time or another...
just my thoughts
Roy Ken