Why does our porta potty smell like rotten eggs after a service?

That rotten egg smell is hydrogen sulfide gas, and it usually means the deodorizer wasn't properly mixed or dosed during the service visit. It's not a problem with the unit itself.

A vacuum truck pumps the waste and then adds fresh deodorizer. If the driver rushes and doesn't add enough deodorizer, or if it doesn't get swirled around to coat the tank walls, bacteria start breaking down waste anaerobically. That process produces hydrogen sulfide, which smells like sulfur. Hot weather speeds this up dramatically.

Call the rental company. They should send a driver back out to re-service it properly. They'll pump a bit out if needed and add a correct dose of deodorizer, making sure it mixes in. It's a quick fix for them. In my experience, this happens sometimes when a route is behind schedule. The unit is fine, it just needs the right chemical balance to control odors.