We have been fighting a gray tank smell in our rig since shortly after purchasing from the original owner nearly 3 years ago. The stink was most noticed in the morning & sometimes while driving down the road. We tried everything, upgraded vents under cabinets made sure the washer/dryer trap was full, insuring a window was open while using the Fantastic Vents etc. The only thing that helped was to pour a small amount of bleach down all the drains on a weekly basis.
Then someone noticed that there was water leaking outside in the wet bay, which just happened to be open that day. Turned out to be dripping while using the shower, which in our case was directly above the wet bay.
Removed a stainless steel panel on the front of the shower tub that was covering a 6" hole for access the shower drain /water lines. Found the metal down pipe/tub connection attached to the ABS P Trap was rusted out & broken.
It gets better. For clearance of the P Trap, the top portion of subflooring was cut away, and since there is another flooring beneath the aluminum joists, it formed a cavity in which insulation was stuffed into. This cavity was water tight forming a pool just below the pipe/trap that had been leaking for a very long time. The overstuffed insulation formed a wick which brought some of the pool water out where it would eventually leak out behind the wet bay between the holding tanks where it would never be noticed.
So after removing the very bad smelling insulation & vacuuming out nearly 2 gallons of the most foul, rancid water you could imagine, the down pipe was replaced.
We were concerned as to why the pipe had rusted out so badly. They usually last much longer, unless subjected to harsh conditions such as high humidity/wetness. Come to find out, one of the abs drain fittings was never glued at time of assembly. This fitting leaked & we think it set up the conditions for the piping to corrode prematurely.
The problem now is that the abs fitting cannot be properly accessed without major surgery from underneath or removing refrigerator & creating a new access. Was able to apply some Flex Tape to the bottom of the pipe & stop the leak until a more permanent repair can be made.
Since the removal, repair & drying out, no more smell...………..
Sorry for being so long-winded about this, but wanted to detail the episode & encourage others, especially those with our vintage/model Beaver, to inspect under the shower, as, where water flows, leaks can occur.
I hope this is an isolated assembly miss.