Sounds like a venting problem. But make sure you use lots of water when you use the toilet especially after a bowel movement. Use the easy dissolving t.p. and make sure you keep the black tank closed so that it fills up and then drain it and do a followup rinse of the drain hose with the grey water. We also rinse the black tank out good at least every month and although some people do not use a chemical, we do. We use the packets from WalMart, and have for about 7 years. Since we use our bathroom area 98% of the time,(the two of us), we find we empty the tanks about every other day. As I said though, we use a lot of water as we have full hookups so I don't worry about it. Also make sure you tell any company you have not to through any personal items or Kleenex down the toilet. That can really gunk it up. Hope you find your problem.
