[Daryles]

Just for clarification, can you share the year and model number of your unit?
Others with the same unit can give you detailed answers.
If you are in freezing temperatures, I highly recommend skirting your unit and putting a small ceramic heater underneath on lowest setting. This will help prevent freezing valves and pipes.
If your temperatures have been very cold, the valve could be frozen. Until you get the whole rig skirted, build a skirt using 2" styrofoam board just around the galley (kitchen) tank area and put a heater in there to warm it up.
If you have cable pulls (like in your convenience center) get some WD40. Put the straw against the joint of the pull wire and spray into the cable as you work the cable in and out.
Unfortunately this won't work if the valve has a rod like the one in the wheel well.
Try pushing in then pulling out, many times quickly on the stuck valve. I have done this when mine are very stiff at the beginning of a season.
If all else fails...
Get a long tube and a hand pump. Run the tube all the way down the drain to the bottom of the tank and pump it out.
You may need to call a mobile tech who may have other ideas to fix this. If you go this route, have him give you a once over on your rig regarding the workings of it and things to watch out for.
Good luck.