Thats about the only thing that will blow the two 40AMP fuses. It only takes one little spark in reverse order to do this. I always make sure the cables are connected correctly by finding the word "NEG" or the symbol "-" on the battery case and make sure this is the terminal going to the trailer frame ground.
You might take a look at the battery fluids in the battery. I see you have a older YR model trailer which probably does NOT have a smart mode converter/charger in it. A single mode converter/charger will put out a constant 13.6VDC which is right on the border line of boiling out battery fluids. This could be a another source of high current being drawn if the battery fluids get below the core level and cause a battery shorted cell...
If you do not have the smart mode converter/charger it is probably a good idea to check your battery fluids on an every couple of weeks basis just to make sure the fluids are not being boiled out...
just a couple of thoughts here...
Roy Ken
