We have struggled with a similar problem and have so far not really solved it. But we did find out one interesting details.
Our black water tank vents through the roof but the vent for our grey water tank is located in the cupboard under the galley sink! We have one vent in the roof and assumed that somewhere that vent had a Y connectiontion to both the black and grey water tanks but a serviceman showed us this vent. It was quite tucked into a corner and we would never have seen it had he not shown it to us.
Have a look to see if you have a similar surprising location. Maybe this helps.
We alos posted this problem on the forum and received a number of very helpful postings. Here below I am posting one of them with credit to George.
Elly
Hi Elly,
Got a few questions about your shower and gray water tank drain problem:
a. Are your kitchen sink, bathroom sink and tub/shower all on the same side of your RV? Yes
1. If yes, is the roof vent on the same side as the facilities? Yes
2. If no, which are on what side? What side is the roof vent on?
b. After you dump the GWT can you see water in the sinks/tub/shower drain P-traps? Yes
c. When you dump the black water tank do you get the same slow drainage? No
d. Has the GWT gate valve been changed recently?
e. Do the sinks drain more slowly than they used to? No but the shower does
f. When you dump the GWT does the discharge sound like a steady stream -- or does it seem to surge? By "surge" I mean a big rush out followed by a slow trickle, then another big rush out. Very slow stream
g. Is this slow drainage something which has happened rather suddenly, or has it always been that way, or has it been worsening slowly? Suddenly starting during walk (when George took shower, it did not drain)
h. Have you tried any kind of drain cleaner (e.g., Draino, etc.) in the shower drain? No
I. Do you have a clear 3" drain hose adapter which allows you to watch the tank discharge? Your eyes might tell you a lot more than what your ears do. NO
What might help troubleshoot the problem is for you/Jack to draw on paper a simple diagram of how your tank system's vent and drain lines are *actually* connected together and to the holding tanks. Then it's a "simple" matter of playing "what if." Keep in mind that water is heavy and wants to rush out of the tank rather quickly. In order to do so the volume of water leaving the tank must be replaced by an equal volume of air -- from somewhere -- at the same time.
The only purposes of a roof vent is to allow for air to enter the tanks when they are dumped, and to escape from the tank as the water level rises. The roof vent has nothing to do with getting rid of tank odors -- IMHO. Depending on how the vent line *and* sink/tub/shower drain lines are configured a blockage in the vent line *could* cause air to be drawn from the sink/tub/shower drains into the GWT during dumping. If that happened the water in the traps would almost disappear completely during the dump. It's not something you would notice unless you specifically looked for it. If after a dump the P-traps have their normal water level I, personally, would not waste a lot of time trying to determine if I had a roof vent line blockage. I'd start looking elsewhere for the problem, again depending on how the vent and drain lines are actually connected together and to the tanks.
A possible cause for slow tank draining could be the vent line going into the GWT has slipped further into the GWT than what it should be. Once the water level goes above the bottom of the vent tube in the tank problems will manifest themselves, such as sinks/tubs/showers slow to drain and the GWT slow to dump.
It's possible you have more than one problem, such as a blockage in the tub drain line from human or pet hair accumulation and/or a restricted vent line, and/or a drain gate valve which is not opening fully, and/or....
Hope this helps. When you find the problem(s) please let us in on the secret.
Best regards, George
P.S. I don't necessarily need or want the answers to the above questions. I offer them as food-for-thought as you try to isolate and identify the problem -- things I would ask myself before I started down any particular path towards the solution. Focusing too closely on a blocked vent line in the beginning may keep you from "seeing" other indicators of the real problem.
