I'd be looking at the Davco plastic dome to make sure there are no small cracks. Next I'd be carefully inspecting fuel lines--hose chafing can cause an air leak without leaking fuel.
I forgot to post how this was fixed. I ended up driving the coach to the Detroit Diesel dealer. Everything was looked at. Davco, fuel lines, etc. - all good. They decided it had to be the fuel tank. They drained the fuel and took the tank off. Inside was a check valve gone bad. I'm not a systems designer, but why would a check valve be inside the tank? Anyway, I hope this helps someone.