I had similar smell coming from my camper about 3 years ago. We were doing work on it next to my garage and plugged it in for power to tools and such. When I tracked down the smell, I took the cover off the battery box, and the battery was swelled out some and was hot to the touch. I unplugged the camper and unhooked the battery and set it aside. I replaced the battery, but I checked other stuff also, and all seemed fine.
Turns out that on the hot wire near the battery box is 2 fuses. One of them had blown, and was causing the battery to be overcharged. That was it, a blown fuse. Haven't had a problem since then, but that is something else I check periodically when I mess around with the camper.
I hope this advice helps anyone along the way and maybe save a potential mess or injury from an exploding battery.