When I first set up I had a trailer system with my DIY streamline box on top of the horizontal tank - and I was using all hozelock type connections. I found I was getting air sucking in between the tank and pump.
I read somewhere on here that hozelock type fittings work best under pressure - and not so well when sucking water through, particularly with a negative head. I changed the fittings between tank and pump to 15mm copper pipe, flexible tap connector fittings and john guest fittings - didn't have any more problems after that.
Since changing to the Van system I used the same pipework to the bigger tank - never had any problems. The great thing about JG fittings is that they can be disconected in seconds if needed.