We were next to someone who also had really bad odor in a new to them used rv. They talked to the septic guy when he came to do the weekly rv pump in the campground. Had him clean tank with higher pressure sprayer, used some chemicals he gave them - came back the next week and used the pressure sprayer again and got all kind of gunk that had been dried in the tank --- $50 for the whole process including the chemicals. I would highly recommend them over and rv place for problems with clogs and odor.