Eldiss Autoquest 145 2011 model water problem

I have a 2011 Eldiss Autoquest 145.  Ever since we have had it (19 months) the water has filled up okay when at home and on some sites.  At approx. 50% of sites it stops filling when only 25-30% full.  The company I purchased this from new have not yet been able to cure the problem.  Because it fills okay at home I do not know if there is still a problem until I try and fill when away.  We are finding this increasingly annoying - has anybody else encountered this?  Has anybody an ideas how to put it right?  My own thoughts are pressure but the site we were at this weekend had what appeared to be good pressure but it cut off when 25% full.  We have previously had the same problem when pumping it in from a water roll which I assume is pumping it in at the required pressure.  Ideas, thoughts please.