3910FB black tank not draining

[andylparish]

Ok, I was suspensions that my rear black tank was not emptying and I have confirmed that. Before we were not using the half bath but a couple times and I just thought maybe there wasn't enough in it to drain much. Yesterday I went to dump and it didn't seem like I got anything out of the 2nd black tank and when I got home I looked down in the toilet and looks like my tank is about half full. Valve pulls easy, but must not be opening.

Question is, where is the valve located in relation to my valve handles for the 2nd black and gray tanks?

These handles are not in the convience center but about 3 feet back in a separate compartment with a small door. How hard is it to drop that section of the under belly?