dsamford,
I just fixed this problem on one of the valves on my outside shower (2011 Cyclone).
Hot worked, cold did not at all. Take incoming hose off of the back of the valve after pulling this section forward out of the UDC. There is a little backflow preventer under the incoming hose nut that was stuck with a couple of years of winter antifreeze residue in mine. I disassembled, cleaned and put back together and it worked fine. Be careful. Very delicate plastic. If it's bust up you can take it out while you get parts and valve/shower will work. Only put the seat in a little and let the hose push it in to correct depth when you hook it back up or it will make the spring too tight and not open up with the 40 psi system pressure. The same setup is all over the waterlines in my trailer and this is not the first to get stuck in the off season. You will find these check valves in the bat, Kitchen sink sprayer and supply lines-even where two hoses join under sink. Hope this helps.
WillyBill