Most of the water drain valves are PVC ball valve types and are prone for handle breaking off. I would use a hair dryer to warm up the valve. Turn/grasp the plastic handle as close to the valve stem as possible, not near the end of the handle or it will snap off. Using the hair dryer try working the valve slowly back and forth to free it up.
Also on our water drain valve, the valve must be completely closed and the handle resting against the valve stop, or it will leak water through the valve, even with a the handle 1/16" or less open and in a few days your fresh water tank will be empty! Also If you fill the water tank with the park water connected to the fresh water plumbing inlet, make sure the other water inlet is open so pressure doesn't build up in the water tank.
John & Debbie
2008 Fleetwood Discovery 40X Full Timers Since Oct 2015