To find the cause, you'll need to crawl around and sniff. I know, not real fun, but it will let you know from where the smells are emanating. We had a toilet with a bad seal on the backside of it. It took almost two days to finally figure that out, and that was only by crawling around and sniffing. I called Thetford and they walked me through some things, gave me an incident number, and said to take ti to the dealer to be replaced.
I also installed the 360 siphon vent caps and I now have absolutely no smells coming into the camper from anywhere, even when I leave the vent open in the bathroom while flushing.
Ben and Doreen
Home Away From Home - 2017 PT Crusader 315RST
TV - 2008 F250 Lariat 4X4 6.4 Diesel
42 nights camping in 2015
61 nights in 2016 (still not enough!)