The sewer system in your RV works the same as in a home, with one exception, there are tanks to hold the contents until ready to dump into a septic or sewer main.
In order for liquids to flow through a pipe, air must enter behind the fluid to prevent a vacuum thus stopping the flow. A simple example of this is filling a drinking straw with liquid, then placing your finger over the end. The contents of the straw stay in place until you remove your finger and release the seal allowing the liquid to flow out the other end.
If your RV is more than 10 years old or if you primarily stay in drier climates and you’ve tried all of the other tricks to eliminate gray tank odor, then consider this. Replace the drain vents that are usually located underneath the sinks. A shower may or may not have its own, usually connected to the bathroom sink if in close proximity of each other.
Inside of the vents, there is a rubber membrane that acts as a valve. Over time these valves can dry rot and not move to close off. (Like the finger on the end of the straw works.) Once these disks loose their elasticity, they stop moving freely and stop functioning properly. This allows for gases to back up (and liquids) and pass out thru the valve opening.
These are easily replaced. I just purchased three for our rig off Amazon for just under $7.00 a piece. Unscrew the old one, then screw in the new one. The hardest part is reaching some of them, but fortunately for me, mine were within easy arms reach.
So, if you’re dealing with gray tank odor and have cleaned your P traps, deodorized the tank and emptied the holding tank completely and you still are getting odors inside the coach…check the condition of the vent valves and replace if the rubber membrane feels dry or hard to the touch.
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