Gray Water Won't Drain!

I have a 5th wheel with an enclosed underbelly as well. Has one of those long handles with cable for operating tank valves. After receiving an unexpected black tank surprise when I removed the cap to drain the tank, I decided to put another valve right at the cap. This way I can prevent another surprise should the cable valve have an issue like it did. If they are partially open or stuck you will have the same thing happen to you. Ordered a valterra valve assembly off amazon and used abs glue to install it. I used a grinder to grind off the little nubs that the cap secures to and then applied the glue to the new valve and installed it. Make sure to use abs glue and not pvc or cpvc. most rvs will use black abs plastic and it says abs on the pipe.