One time we had a bad egg / sulphur smell for several weeks. Tried everything, emptying the tanks, flushing them, several different types of chemicals, GEO method, ice cubes, etc. Finally I had a friend come over with a better nose than mine.
After going around the MH for an hour smelling and sniffing everything to include the top vents, we found that my inside, no maintenance, starting battery had cracked and were then overcharging. It was a two year old Decka. I certainly didn't expect to find that. I put in two new starting batteries and no more bad sewage orders after that.
Bill