Sewer Gas Odor

[CT Wanderer]

Our 305RL is three years old now. This year I noticed a sewer smell, (gray water?, according to my nose). Seems to come and go. When tanks are empty smell mostly goes away. But I have detected the odor even with the tanks empty in our yard. Tried deodorizer in the gray tanks without success. I just start using Happy Campers Holding Tank Treatment this year but can't say if I noticed the problem before I used up my old treatment. Decided to investigate further. Dropped the Coroplast and found water between the Coroplast and the silver bubble wrap "insulation". The water had no smell. Suspect this is coming in when driving down the road in a rain. There was no water on top of the silver layer so nothing was leaking from above. Checked piping thinking I may have a sewer gas leak there. Everything looks ok but hard to tell if any of the joints are leaking, they all look good. Got it all open and I can still detect the smell all be it very faint.
Has anyone had this experience and found the problem/solution?