As Oregon stated a disconnect is the best bet to make it easier, safer, and less chance of arching, but if you prefer to disconnect battery cables it is always best to remove the + first to avoid the spark, much the same when using jumper cables for starting a dead battery vehicle. I was taught this many yes ago from my father.
These little sparks always seem to happen (unless you have a battery disconnect switch), and should be minor with all 12 volt DC loads turned off. The only 12 volt DC loads you should have present are the propane detector, the logic circuit board in the refrigerator, and the logic boards in the furnace/ac and water heater. The electric brakes are directly wired in, too, but should draw no current unless the emergency braking lanyard is pulled. A simple DC circuit being a loop, an arc is going to happen at either the positive or negative terminal when it is disconnected.
The main reason I was told to remove the negative terminal first has to do more with working on autos or trucks, but is applicable to trailers too. If you are removing the POSITIVE TERMINAL FIRST with a wrench, and that wrench touches exposed chassis metal ("grounded" - actually connected to the battery negative terminal), you will have a DEAD SHORT ACROSS THE BATTERY and you and your metal tool will learn all about electrical arc welding, with the possibility of a battery explosion with all the shrapnel and flying sulfuric acid. Removing the black, chassis connected terminal first removes any possibility of this chassis metal battery dead short occurring. Remember that the SAME LARGE ELECTRICAL ARC/BATTERY EXPLOSION possibility exists when re-connecting the battery. RE-CONNECT THE POSITIVE TERMINAL FIRST (chassis metal is not yet connected to the battery), THEN THE NEGATIVE TERMINAL LAST.
