Thetford toilet sewer smell

I had same problem with Aqua magic V toilet. I did the flushing of tank two years ago, did not help. Changed floor seal last year, thought that fixed it. Then got the smell again this year. I read about that there is a seal on the back of the toilet near top that tends to leak after a few years. It can not be replaced. I went to Amazon bought a new Aqua Magi V for around $120 dollars and a Cyclone vent cover for around $20. Less than two hours to install both. That was in the spring of this year and no smells... fingers crossed.