Stearn, you're right about the check valve. I didn't need to replace it. On the Journey (and probably most other RVs) there's an access panel at the bottom back side of the HW heater held with 4 screws. After removing the panel I tapped the valve with a screw driver handle making it work.
Btw, there's no electric selinoid in the system. For you Winnie owners who are interested, look at the plumbing diagrams (a section in the Winnebago forums) for a view of the way things are laid out.