I can not get my gray water to empty. I know the sensors are not always right. But after a couple of days of showers and I have emptied it never gets to "empty" on the guage.It shows 2/3rds full. I get leaking from the underbelly like it is coming out of the overflow. The last campsite it gushed out of the overflow after emptying the tank,(which obviously wasn't empty. It ran all on the ground as I drove down the hill to the campground parking lot. I think it is clogged somehow and need to REALLY empty it.
As Dano said, you have a leak somewhere, and from the sounds of it, it's in the drain line. If it's leaking after you try to drain it, the leak is most likely between the drain valve and the terminal end.
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!