Trouble with grey tank dumping using garden hose

Air must enter for water to run out. While the tank valve is open air enters one of the tank vents, under the sink or through the roof. If you don’t let the tank drain completely the water in the pipes won't drain out, it won’t have any vent. My clear elbow has the fitting to hook a hose to for flushing the line, works great for letting air in to drain the lines.

The "valve" is actually called an "Air Admittance Valve"....AAV. It can be found under the sink (kitchen or bath...maybe both) and can be replaced with one that you can find at just about any hardware store. The valve will unscrew easily.

Ron

This is the kind of "how the damn thing works" description I have scoured the internet for! Thanks!

So you have 2 grey tanks????
One for shower/bathroom sink and one for kitchen sink?

IF so.......

Your shower/bathroom sink grey tank VENT to roof is blocked/obstructed.

The 'valves' under sink (bathroom should have one also) are to prevent sink 'P' traps from getting sucked dry when draining grey waste tanks.
Each waste holding tank has a vent to roof to allow air in/out of tank when filling/draining.

IF vent is plugged tank will struggle to drain due to lack of venting.

Need to clear vent line.
Open tank drain....go up on roof & remove roof capon vent line.....stick garden hose down vent line........blast away with garden hose

Wow, quick replies from both of you. Thank you very much.
I didn't know there was a valve under the bathroom sink, so it gave me the idea to try draining the kitchen sink tank first. Even though the shower water was still visible thru the clear drain thingy, as soon as I pulled the kitchen valve for a few seconds, ALL water drained. I verified by pulling the shower valve again briefly and it drained fine. I don't know how draining the sink first worked, but it seems to.
Thanks again.

Something has to be plugged. Roof vent, check valve under sink. Or you forgot to pull the dump valve with hose hooked up.

You may have a grey vent under bathroom sink. It may be clogged.