Smells like bad fridge but no other symptoms

Subject is a 6 week old Class C. We have been camping continuously in it for the past month or so. We drove it to town for groceries. It was parked in a flat Wal-Mart lot, close enough to level for sure. On the way home we kept wondering what in the area smelled like rotten eggs. That's as close as I can describe it, like SO2.

Back at the CG, we realized the smell was inside the rig, not out in the environment. I plugged into shore power. We can't notice the smell now. Although, out of concern, I shut the fridge off a few minutes ago. But it did seem to be going away.

Before turning it off, I took the exterior panel off the fridge compartment. No smell, no yellow, no nothing. Inside the fridge, everything is likewise looking fine. Temps were quite cold in both compartments. Other than smell, have I looked for all the possible clues for a fridge going bad?

It doesn't smell quite like batteries gassing. But not all that different either. It's possible. Like the poster of a month ago with a similar situation, it's thunderstorming right now, but in AZ instead of FL, lol. So I can't check the batteries. It does not smell like grey or waste water, or pet urine. Anyway, focusing on the fridge for now, what else can I do to check it or watch it? We'd like to keep using it, obviously, if it is not the problem.