Shower WOnt drain if Gray valve is shut

[Cmccain13]

2021 Coachmen leprechaun 300bh
Ok so i have been looking at a lot of things on here and I cant seem to find the right answer. ( Preface, my motor home is a bit off level to the front) So first off I can turn on my sink faucets and run them for a good three minutes and no back ups when I turn them off there is a significant amount of gurgling coming from my walls. From the looks of it that is where the vent pipe is located. I was taking a shower and water was draining and then all of the sudden nothing, i mean not a drop more. i went to the roof to see if the vent was clogged and when i looked in there i could see water and it was bubbling. so i opened the valve and it all went away. so im not sure why water would be going up the vent pipe. I'm a first time RVer and need some help.

[Matt_Colie]

Cmccain,

What valve did you open?

Is it possible that the gray tank was full to the level of the shower tray"

I don't know Coachman well at all so maybe someone with more familiarity can be more help.

Matt