Why does our porta potty smell like rotten eggs after rain?

That rotten egg smell after rain is almost always rainwater getting into the holding tank and watering down the chemical, so the waste goes septic faster and starts putting off sulfur gas. On my remodels I've traced it to two spots. First is the vent stack: if the cap on top is cracked or missing, rain runs straight down the pipe into the tank. Second is the roof seam or the gap where the vent passes through the roof. Stand inside after a storm with the door shut and look up. Daylight or drips means water is getting in.

The other half is load. About 20 guys on one unit for a week is right at the line OSHA 1926.51 sets for a single toilet, so that tank is pretty full by the end of the week anyway. Add rain on top of that and there's way less deodorizer working per gallon of everything in there.

What fixed it for us was asking the driver to check the vent cap and roof on the next visit and add a chemical top off after heavy rain. If midweek looks wet, ask about a second service that week or a second unit. Swapping to a different unit type won't solve it if the tank is still taking on water.