Originally Posted by 53kats
We have a 2011 30 ft Airstream Serenity. No matter what level the grey tanks is at when we empty the kitchen sink the water backs up into the shower and then drains. Could this be a vent pipe problem? Any ideas would be appreciated.
Maybe, there is a venting issue /problem.
The parts manual for your trailer has a schematic drawing of the waste water system, as it should have been installed in your trailer. According to that drawing the pipe for the shower and the pipe for the kitchen sink are separate, not connected into a single drain pipe. Look on page IV-48 at this web link:
If it is plumbed this way, water would have to be pushed up out of the gray tank into the shower. I'll bet there was a change during the construction process that combined those two drains. I would ask Airstream if they can provide the shop drawing for the actual plumbing layout in your specific trailer. Or, look to see if you can confirm how it was done. If there was a change in the layout from what is shown in the parts manual that combined the sink and shower drain, that drawing might reveal the issue.
In the meantime: look under the kitchen cabinets for an air admittance valve on the sink drain pipe (google "Studor Mini Vent"). These valves have a rubber diaphragm inside that can get stuck, not allowing the fixture to vent properly. Temporarily remove it. If that solves the problem you know the valve is bad. If water comes out at the valve location when draining the sink, you know there is a partial blockage beyond (see last item at end of list below). There is also another one of these on the shower drain, that should be beneath a closet or under a bed. These valves are not hard to replace. They are shown on the schematic noted above.
Another thing that can cause this vent problem: if the main vent from the gray tank was not originally supported properly it can slip downward into the tank. If the bottom of this main vent pipe gets below the water level, water from a high fixture can be forced back into the drain system into the lowest fixture (similar to your description) because air cannot vent out the main vent from the tank.
Another possibility, if the pipes were combined: a partial blockage (hair ball) in the drain pipe between the combining point of the pipes and the tank.
Good luck figuring it out!