slow draining shower

05 2900 dual slide. The shower is slow draining, unless the gray water tank valve is open and draining. Seems like a venting problem, but I can't figure out how to fix it. I do have a vent hose going to the drain pipe, which goes up to the roof. That will only vent when the drain pipe is used to drain the tanks.
I don't see any kind of venting on the gray tank itself though. The black tank has its own vent pipe.
I am pretty sure that the whole tank system has been replaced at some point. I do not have the electric valving anymore. How do I get a vent into the gray tank?

thanks
Denny P

I have a 2012 PC 3100 and my drain was also slow. It was just gunk buildup in the drain. This is an easy way to clean it out and will do no damage to the plumbing. Take the strainer off the drain. Run hot water into the drain. Poor a cup of baking soda into the drain followed by a cup of cider vinegar. Let stand for about 30 minutes. Flush with hot water. Repeat if needed. Worked for me.

Well, my slow draining shower ended up being a vent issue. Someone in the past changed the holding tanks to a manual valve setup. But in doing so, they didn't put a vent in the gray water tank. I proved my theory by drilling a 1/4" hole in the location where I will put the flange fitting for the hose and now the shower drains without issue.
Next project will be to get a hose on it and position the hose up the vent hole with the other vent line for t he black tank.
Another mystery solved...