Any rv techs out there?

I think I figured it out. The excess flow check valve acts like its stuck or gummed up. Using compressed air I can only get flow from the outlet side to the inlet side of the hose as long as I take the back flow (that's what i'm calling it) check valve out.

I have been reading up on the excess flow check valve and it looks like they are very susceptible to contamination. This camper sat on the lot for a while with no cylinders, so that may explain it.

Cant take it back to the dealer. This is kind of a goofy situation. The camper is a 2012 model however a couple purchased it and brought it back the next day because their pickup couldn't pull it. So we ended up getting a hell of a deal but no warranty because we are the second owners.