I know of basically 4 things
First, as you know reversal of the battery leads
Second A short, if he shorted the leads WITH THE CONVERTER ON (plugged in) it is possible in some cases for a converter to out just enough to pop those fuses.. Usually this indicates the wrong fuse used (IE: 2 30 amps will blow if your converter is an 80 amp, 3 won't, or 2 40s)
That short can be in 3 places.. It can be a bad circuit in the RV, or it could be shorted cells (Plural) in the battery, or it could be an accident during battery removal/replacement as described above.
(I counted it twice in my 4 both shorts in the RV and accidental install goof count as one, Shorted cells as other)
Finally Fuses sometimes fail, for no describable reason, Vibration and age is one theory,
