As others have said, assuming that the valve has been open all this time, you'll need to break up the solids in the tank.
I would start with as much HOT water as you can add, like a 5gal bucket, with a good quantity of dish soap added, let it sit for 10mins or so, then drain completely (meaning whatever will come out) and then repeat. The hot water will help to soften the "deposits" in the tank, and after several buckets of hot water, things should start to soften up and move. Might take a day to get it moving. Don't know if you have access to the black tank from underneath the trailer, but if you do, I'd put an electric space heater down there blowing up on the bottom of the tank. The warmer you get the tank and it's contents, the faster things will start to move.
If you need to make a "spray wand", take a standard garden hose with a hand sprayer on the end, tape the nozzle to a broom handle or some other stick, then tape the lever completely open. You can then lower the nozzle into the tank, turn on the water, and spray. That could help break things up as well.
Good luck!...sounds messy...but it's actually not too bad. Just take your time and keep working at it, and it will eventually break up and flush.
Then CLOSE THE VALVE ;) for normal use until you need to dump.
Jim