Last trip or two to the lake I've noticed that on the way home at the end of the trip I get the sulfur smell in the coach. No odor problems whatsoever with an empty tank, or while in the campground, only with a mostly full tank going home. (Our local lake is a state park without sewer hookups and I hate waiting in line at the dump site, so I travel the 30 minute trip home and use the sewer cleanout connection beside my driveway to dump my tanks. I always thoroughly rinse with the built-in rinser, and when done with that, I add 2 gallons of hot water mixed with Dawn detergent back to the black tank to sit until the next time. I also add one bottle of the enzyme liquid at the start of each trip.
What gives?