Sounds like something is wrong with that valve. Do you have the Pex crimp connections? You can buy a crimping tool for $50, some crimps and a new valve for a couple dollars more... or you can buy some Sharkbite 3 way valve with shutoff for about $20 and replace the valve yourself.
Pex is actually very easy to work with. I have used the Sharkbite style fittings on several applications and they have been problem free and super easy to use. A little pricey, but cheaper than a plumber or RV repairman!
Good luck!