Charging of leisure battery on the caravan

Can anyone please help

I have recently allowed my leisure battery to run down while on a site with no electric hook up, when we got back I connected the power to recharge. Next morning I went to check all OK but found a bad smell in the van and when I opened the battery box I found condensation (or acid) every where and the battery red hot. I have now disconnected the battery and cleaned the area, plus purchased another battery, BUT I'm at a loss to understand why this should have happened.

I feel quite certain if I had not gone back to my Van there would have been an explosion or fire, I'm now afraid to leave power on the van when not in use.

Any good advice would be welcome