I always look for the word NEG or the symbol "-" on the battery case and make sure this is the terminal that is connected to the trailer FRAME GROUND.
The RV Trailers use the house hold wiring schemes which is WHITE is GROUND and BLACK is HOT. However the Automotive world uses RED as hot and BLACK as frame ground. Since a battery is used in both situations you cannot be sure what is what unless you look for the battery terminal case markings...
It certainly sounds like you have reversed the connections.
You also have an older trailer and who knows what the previous owner has wired up for you haha...
There is no rules that one must follow code. Its all on the HONOR SYSTEM hehe...
My 12VDC wiring in my 2008 was all white wires but the 12VDC + POSITIVE wires all had a color stripe on them and the solid white ones were 12VDC - NEGATIVE wires.
The RV and Marine Boat world also has batteries with the same physical connectors being used for both POSITIVE and NEGATIVE cables at the battery side using the Ring Terminals...
The Automotive world has a large hole and smaller hole in the two battery posts terminal connectors on the battery to give you a clue haha...
I guess it is all this way to keep us on our toes all the time...
You will need to locate the REVERSE POLARITY fuses and the In-line fuse close to the battery positive terminal and get all of those replaced and make sure you hook up the cables in the correct polarity otherwise it will just blow them again. That is what all of those initial sparing was doing in your opening remarks.
When you get it all back to working again then if I was you I would use some red finger nail polish and mark all of the cable ends that are suppose to go to the positive terminal of the battery...
some folks like to take photos with their cell phone camera and save the photos on their cell phone to keep a good record how they are suppose to be hooked up...
I blew up my fuses one time with replacing on of my 12VDC batteries with a new one that was a couple years newer and the manufacturer had reversed the terminals around on the top for the same battery type. Without looking at them I hooked them like the other like batteries in parallel and one little spark killed all those fuses discussed here. They went POP... Then I knew right away what I had done hehe...
Thats when I started always looking for the battery case markings first thing...
Roy Ken
My Posts are IMHO based on my experiences - Words in CAPS does not mean I am shouting
Roy - Carolyn
RETIRED DOAF/DON/DOD/CONTR RADIO TECH (42yrs)
K9PHT (Since 1957) 146.52M
2010 F150, 5.4,3:73 Gears,SCab
2008 Starcraft 14RT EU2000i GEN
2005 Flagstaff 8528RESS