When we were on vacation from time to time we would get the smell of sewer gas in the POD, I always put deodorizer in both tanks after dumping them and even though the sewer hose stays connected the whole time the valves are always closed. One morning the smell was quite strong after dumping the gray water tank and it did not seem to drain very fast, almost like the vent was blocked. I could smell odor coming from the vent under the bathroom, after removing the vent cover and looking inside I could not see anything loose or leaking.
Fast forward to this last weekend and now that the trailer is empty in the storage area under the bed so it is easy to get under there to remove the panel at the back of the toilet to check all of the plumbing connections. Everything was tight, so up to the roof and take off the vent cap to make sure that there were no blockages. What I found was a large gap around the vent pipe where it comes through the roof, I believe that the odor was coming from the vent pipe and instead of going out into the outside air it was dropping around the pipe, through the gap and would drop to the floor area under the bathroom coming out of the floor vent into the POD.
The fix was to seal the gap around the vent pipe at the roof with the only thing that I had on hand, clear silicon sealer. Now I have to wait until next spring to see if the fix worked. Goose