fresh water tank possible vent blockage

I bought it new in 2004. When we dry camped until 2009 I never had a problem. And no problems until our last trip a month ago and thought I just forgot to fill after we got home but it drained out last week and I thought it stopped because the pressure was gone but today I noticed the level indicator on the panel showed 1/4 full. So I refilled it and it drained out the overflow for an hour before it stopped at 1/4 full. But this year it started syphoning out. I fill it and store it until the next trip a month apart. It used to expand from pressure and excess water would flow out overflow outlet for about 5-10 minutes. the overflow opening has a rubber flapper type valve for one direction flow out and seals itself so it can't suck anything in. I store it here at home and go in it often and I hear the overflow valve fluttering.