[DutchmenSport]

In this case, I don't think a Flush King would be much help. If the black tank is full, there is a lot of pressure on the tank slide valve already from the volume of water in the tank. There is simply not enough water or force on the other side of that valve to make it greater than the force that exists on the inside of the tank right now.

Actually, about the only solution, ..... and this is going to be horribly messy .... is to remove the end cap off outside discharge port, open the black tank valve and shove something up the pipe, .... like a garden hose, soft, flexible, but firm. I would not use a metal plumbers "snake". They can cause more damage than good.

Simply shove the hose up the pipe. If the valve itself is frozen shut, or stuck shut, or the cable from the pull handle to the valve itself to the slide valve on the tank is broken, then you'll never be able to get the hose any further.

If it is something blocking the valve from the inside ... like construction material from when the camper was made, then you should be able to push the blockage away with the hose and start flowing again.

BUT! Beware ... all that black tank content will come rushing out. You'll have one heck of a mess on the ground (and on you) that will need to be cleaned up.

If you have a toilet paper clog at that valve, the hose should be enough to punch a hole through it too. Just be prepared when the bowels of the trailer break loose!