UPDATE: OK, the problem still exists, but I think I have an answer... The drain valve is NOT the problem, it is working fine.
I tried running a hose up into the tank from the drain and blasting it loose with water, but that didn't work. I tried using water pressure from inside the coach too, but not being able to block all the sink traps that didn't work, it just forced water into the sinks. From outside, I also tried using air pressure to break through, but nothing there either. I then tried a plumbing snake and ran that into the tank, both from outside and from inside. I did get a small amount of hairy gunk from inside, but never dislodged whatever is blocking the tank.
I then visited a local RV Parts Store looking for additional help and after discussing the problem the parts guy told me that it's probably the build up of scummy stuff inside the tank. He thinks it's broken loose from the bottom of the tank and is now floating around, blocking the drain pipe. Apparently the stuff is like a plastic sheet, mostly made of soap scum. That would explain why all my fix attempts didn't work.
My last resort, is to pour an enzyme gunk eater into the tank and pray. Please pray with me guys, and thanks for all the help and suggestions! Always nice to know you are there!!!
Loren