Air lock in water system

Our caravan has an onboard water tank, for the last few outings we have had "spluttering" & sometimes no flow from the taps. the taps are mixers ( kitchen, handbasi & shower)  we open them all, on the middle setting until they flow freely and never let the tank get too low but a while later the problem re-occurs. Any ideas ?  we have had the van 2 years but this has only recently started.

thanks

Glyn