Thetford toilet sewer smell

I have the same toilet model. I suggest if you have never tried one, buy a flush king and backflush the tank several times. It's the best thing I've found for black tanks. Gets all the gunk, paper etc out of there which causes smell. Toilet tank treatments are a waste of money. Always keep water in the black tank and make sure there's water in the toilet bowl. The tank vent on the roof as suggested previously is a good thing to check also.