8Several correct answers above. I will capitalize it...
YOU HAVE A BAD GROUND CABLE OR CONNECTION BETWEEN THE ENGINE STARTING BATTERY AND ENGINE BLOCK. "BAD" as in cause-a-fire-bad.
That cable is SEVENTEEN YEARS OLD. Forget trying to fix it. Go to the auto parts store and replace the cable with one that is one size LARGER in thickness.
NEXT, double up on that ground strap that burned. Add a second one. Engine to chassis then another link to the sheet metal body. On motor homes I used to use FOUR GAUGE. Six gauge will do fine as well.