The trick on checking batteries for proper terminal connections it to always find the stamped "NEG" or "-" stamped into the battery case and make sure this is the cable that is going to frame ground real close to the battery. Using the colors of cables will get you in trouble as some color schemes have BLACK for positive with WHITE for ground and then it might be RED for positive and BLACK for grounds. So the best thing to is look for where the NEGATIVE GROUND cable is connected to the trailer FRAME GROUND and make sure this cable goes to the NEGATIVE terminal of the battery. Once it is correct alot of folks will mark the battery cables with something to identify which is which. Dont get use to the way they are connected on the battery as the next battery you may purchase even from the same battery company may have the bettery terminals in a different location on the battery - Just look for the stamped "NEG" or "-" and you will always be ok.
As you found out it only take a little spark when you hook them up backwards. This is most cases blows three fuses.
Two will be on the Power Distribution Panel board where all the fuses are located and will be labeled "REV POLARITY" or something like that. There will also be a IN-LINE real close to where the battery is installed. This may be right next to the battery terminal cable or maybe just under the trailer frame where the 7-way trailer cable goes to. I have seen some inside the electrical box where the 7-way cables wires are connected to the trailer chassis wires too.
It is always a good idea to have a small inexpensive digital multimeter in the RV TOOL BOX. This is available from LOWES, WALMART, AMAZON and any number of auto parts stores. It only cost $7-$20 and really takes away the quessing where you have lost power. Looks similar to this.
photo from google images