Mark,
Ive had a similiar problem in the past and found that the connectors weren't fitting as snuggly as they used to and weren't engaging the non return valve.
I just snipped off the valve and all was well, although with yours being on a van port you might want to replace the connectors as it would probably syphon your tank if you don't!
To test if it is this turn on your controller, plug the hose into your vanport and firmly hold the connector against the port - if this solves the problem it is the non return valve.
Hope this helps
Regards
Al