The PEX valve is cheap. It's the PEX installation tool that's a killer!
If you're feeling adventurous, take down the bellypan on that side. Follow that blue PEX line back to the fresh water tank's vent outlet. Splice in a loop of flex hose from the tank, back up through the floor and then back down again. That breaks the siphon and then you can fill the tank up all the way.
That's all that Redwood did when I had it back to the Service Center for this problem.
Mike & Pam '12 36RL, Kodiak Discs, 17.5" G114s H-Rated, SRE4000 w/HD Shackles, Joy Rider Shocks, Flex Air, 5.5 Onan, W&D, DW, 4 Scare Lights, (2) 6V Trojan T-145s; Progressive EMS-HW50C, Winegard DTV Sat, Sony/Denon 4K Home Theater, WiFiRanger EliteAC Pack, weBoost 4G-X Cell Booster.
2020 RAM 3500 Limited CC, LB, 4x4, DRW, Max Tow, Cummins HO, AISIN, 4.10, Mopar 30K w/OEM Prep, Transfer Flow 70gal Aux Tank/Toolbox.