Water level on the batteries was fine. It's not propane or septic, those are different smells and I'm sure I'd be able to tell what those are.
Here's my current theory :
Running fridge on battery / propane for 48 hours continuous depleted the battery charge state such that when I got back in the rig to leave, the charging unit (whatever that may be) said "oh wow, batteries are down, better charge them up pronto !" and began to charge them at a rate higher than is prudent. Caused *some* overheating / boil off / sulfur smell until the batteries charge level came up then the charger calmed down and the smell dissipated.
Thoughts ?
This is the only theory I can come up with that matches the fact that everything in the rig is still working fine. Batteries (2x 6 volt) are showing fully charged, fridge is still working fine and there isn't an smell anywhere now.