Forgive me for asking, but is it stuck or do you just not know how to close it? If the valve is physically broken/stuck, about the only other way to stop water from running out is to somehow put a stopper on the end of the drain line. Some may have threads so a plastic or metal pipe cap can be put on - a hardware store will have something that fits. If no threads on the end of the drain line but it is solid pipe, there are slip-fit pipe cpas as well, but they have to be cemented in place with an adhesive. Make sure you use a temporary adhesive or that you can cut it off when no longer needed.
If it's a flexible rubber hose, you can pinch shut with vise grips or similar tool, or maybe insert a plug of some sort. A hardware store will have friction-fit plugs that have "hose barbs" (ridges) to prevent them from popping out. I've also done temp plugs by whittling a piece of wood to fit inside and holding it in place with a hose clamp.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL