I (actually we - with Vinnie..) just replaced frozen version on ours.. The plastic does deteriorate, and it will gradually freeze in position or handle will break off. Two screws easy to remove, but inside, probably behind a small square inspection plate with rusty screws holding it in place) is a small plastic hose connecting the cast tube on the back of the valve to the tank outlet, with a pair of PEX clamps.. There are three versions of that valve with different OD's on tube on back, but if you get matching one it may be 5/8" ID tubing about 4" long with a pair of clamps to fix it..
Sorry no pictures, but it was a hot day and drain valve was bonus repair in addition to replacing both axles and shocks... If you see a small square plate anywhere near drain valve on belly held on by 6 or 7 small screws, remove that to get better view of what you are going to need..
In Theory, there's no difference between Theory and Practice, but in Practice, there is usually a difference...