After almost 2 years of intermittent grey tank odor penetrating the cabin we finally solved the issue: a faulty joint at a coupler just under the roof vent. The odor always seemed to be strong next to the shower wall in the bedroom (2006 QTH floorplan). By pulling the heat vents and plumbing access panel I was able to get a light, mirror and eyes into the void behind the wall. No obvious answers until we used compressed air at the roof vent to chase the source. I noticed wood fibers falling from up top in the wall cavity when my helper induced the positive (slight so as not to blow traps or vents). I could see a coupler on the pvc tank vent pipe just under the roof and the joint looked suspect. The odor was strong in the wall with the air we were pushing into the tank. All other possibilities had been exhausted so we pulled the plywood skin off the wall to get to the pipe and I could feel a strong stream of air coming from the joint. Same air that had dislodged the wood particles left by the hole saw during the original build. The joint was still solidly glued but allowing air/gas to pass. Some 3m 5200 marine sealant took care of what I have to assume was a sloppy glue joint by the factory. Finally we have a stink-free coach and can put up all the candles, air fresheners, etc!
Hopefully no one else will experience this problem but I wanted to share just in case. Taking the thin plywood panel out was not that difficult. It was shot in with staples and removal, repairs, replace only took about 2 hours. Still not fun to rip into a coach like that!