You said you removed the underbelly to see the dump valve. It's one of those remote cable operated ones, then...? Our Jayco has them and I don't like them. Are you in a location (and in a mood!!) where you could loosen the four valve mounting screws/nuts and see if it leaks some drainage?
Can you get it to a dump station where you could jam a running hose up the dump opening, to and through the valve? Measure the run to the valve so you can be sure you pushed enough hose to get to and through it.
I really find it hard to believe that you can't flush it loose with a hose from outside IF IF IF the valve is open.
Station an observer at the toilet to be sure you don't overflow it.
