I started to smell this rotten egg smell and after a lot of crawling around I traced it to the chassis battery. It was really hot and giving off that terrible smell. I disconnected it and took it out ...
Batteries fail. Generally 5 yrs is max but some 6VDC last longer. Anyway, my guess is your battery failed internally and cooked itself. Follow your manual's advice. Check your batteries monthly to ensure water levels are up. Also, if you have a battery bank, one weak battery will drag the others with it....Dennis