Happened to me a few years back in the 4905. I had two group 31's back near the engine, and I just used a Dayton 24v automatic charger with clips to keep the batteries charged while parked for the winter.
I would start it once a month, and let it warm up to temp, build air and exercise things. One morning I decided to get that out of the way, and the moment I hit the starter: BAM! Battery gassed from the load, clip ons sparked and the rest was a mess. The feel at the front of the bus was that the crankshaft had busted in two!
Funny part was, the guy next to me in a fifth wheel trailer came running out in his underware screaming for everyone to get away, because his water heater had just blown up and was probably on fire.! He got into an arguement with the guy in a 4106 on the other side of me, who maintained his BBQ tank had just blown up. It sure was hard to keep a straight face while they almost came to blows over whose RV had just exploded.