Hi,
Is there a leak of water anywhere? ( If there is no leak it doesn't necessarily mean it's airtight) I would look at the tank outlet first. You will have a reducer screwed into it to change the female thread to a male fitting the hose will push on to. I would try some silicone on your finger all the way round the male fitting's thread. I would try to work it into the thread and leave it to cure. Then see if that works. If not, then its got to be the hose couplings or maybe a tiny hole in the pipe itself.
We also have a static system but both our pumps have been fitted with inlet filters/strainers to stop any dirt, accidently introduced into the van tank, from getting into the pumps.
Spruce