Water overflow from tanks

I have a 5 year old compass/road star van when I connect the water hose to the van while at a caravan park the water bypasses somehow and slowly fills the water tanks once there full I get a continual dripping of water around the filler cap off the water tanks, I have tried to find out how it is by passing but its got me beat , never used to happen when the van was new so I am guessing there must be a non return valve or something like that that's leaking??

Bruce Putland, Vic