I'm no expert but I think it's a fairly common problem. Probably the valve is bad or an obstruction. Nothing can be more aggravating. Put that additional valve where the cap goes. That's what I've done and I see some of the new campers are coming online with the one final valve at the cap. That extra valve won't fix it but will offer some additional protection until you can get it fixed. Good luck.
"It's a good thing we don't get all the government we pay for.":)
~ Will Rogers ~
Priscilla, best friend & navigator.
Hank (5 year old black & tan Dachshund)
Cocoa (3 year old brown & tan dapple Dachshund)
2010 Fleetwood Excursion 30a
Toad is a 2007 Jeep Wrangler X
(We traded in our F250 and Jayco Eagle)