Trouble with grey tank dumping using garden hose

Swapping which grey tank drain valve was open should not have 'cured' your draining issue with the shower grey tank.

What may have happened........
Shower grey tank vent to roof sticks to far down into grey tank and when water level was high enough it blocked the vent.
Then when you tried to drain thru small garden hose it became 'vapor locked' (no air coming back in vent because it was under water)

Then when you swapped drain valves some water transferred to galley tank grey tank for kitchen sink) uncovering shower grey tank vent line.

Fill shower grey tank full and try draining again.......if it doesn't drain then vent line is getting blocked by water level in tank.

My sound like a dumb question, but does your garden hose have the screened type rubber gasket? It may be plugged.

(Having some trouble replying to specific people, so thanks to all for your help. Here's the outcome...)

Both grey tanks will now drain properly thru the garden hose. I don't know for sure what worked, but I know it wasn't the fault of the hose.
The roof vent caps looked fine. I don't think anything could have gotten in them. It seems alternating the Kitchen/Shower valves somehow allowed the replacement air into the system. It's really the only thing I did that could have had an effect.

Again, thanks to all, especially those who helped me understand the inner workings of the holding tank systems.

Are you talking about the clear type adapter you install on your RV drain piping?

No. It's a drain cap with a garden hose adapter fitting.

Is your garden hose clear?? It could be collapsed inside, if it has kink and stayed that way for a time, the inside line may be loose. Under pressure it won't have any effect, but gravity flow would be restricted. Are you sure it is going downhill all the way??

Air usually comes from up above the roof (there should be 2 or more vents up there,, They look like a pipe wearing a hat). Sounds like it is either:

A: Clogged (run an electrician's fish tape or a plumber's snake down it, Be advised that they do not always run STRAIGHT down into the tank, sometimes they have elbows in the line (usually 30 or 45 degree ones) to offset around an obstruction or bad planning)

B: Pipe has fallen down into the tank to the point where it's efectively clogged.

It sounds like we don't have the same system (we have one grey tank), but I have found that sometimes when I attach a garden hose to the hose adapter on the sewer cap it simply can take awhile for the grey tank to drain.

AM Allan wrote:
I can't get grey water to come out of a garden hose when attached to my
adapter. Help!

Are you talking about the clear type adapter you install on your RV drain piping? Like this


If so, this garden hose connection is one way type and used for putting water into the sewer line, not for draining water out of the line.

Good luck.

Lakeside

As long as there is a roof vent for the tank the air admittance valve shouldn't keep the tank from emptying. The air admittance valve keeps the p-traps from getting sucked empty when water goes down the drain to the holding tank.

Try a shorter hose.