Let the water syphon through first before you switch the pump/s on.
I’m assuming your unplugging the hose reel and leaving said pipe hanging outside the van?
Wait until the water is coming through (you may have to suck it a bit to help it along) before you switch the pump on.
Once the water is coming through, switch the pump on and ramp it up, may take a few mins but it will kick in.
(Added - just remembered you have a boiler inbetween pump and reel do you will have to suck the water to help the water come through as the water will have to travel up before it comes back down.
Once its coming out the pipe then it’ll just keep doing so unaided and you can then just switch the pump on and ramp it up)