If you're using half inch hose on the tank side of your pumps chaps, try switching it to minibore 8mm ID hose and see how you get on. It makes a huge difference on the trolleys. I don't see why it wouldn't work for van setups as well, as they're set up the same as trolleys, just bigger.
The problem often occurs because there is considerable volume to fill in that section of hose when it ends up full of air, and the pump can't suck water up it fast enough to prevent a bubble of air remainig in the top of the hose where it exits the tank. By reducing the internal bore of that section of hose to a level the pump can cope wth, it will prime quickly leaving no air in the hose and provided you have an airtight connection at the pump, the pump will also prime quickly. Whilst priming your system I would also suggest disconnecting your reel if you don't already, so the pump doesn't have to cope with back pressure at the same time.