How about bolting the reel to the van floor, just inside the back door. Have a plywood platform also bolted to the floor with a hinged section that allows it to be folded up behind the door when its closed, and folds down horizontal so sticks out beyond the door when it's open. On the end of the folding bit is one of those "multidirectional hose guide thingies". You would need some simple stop arrangement so it didn't fold down beyond the horizontal.
End of hose is always kept ready fed through the guide.
Stop van - open door - fold down ply section which takes hose beyond the door before needing to change direction - off you go!!
Might be a bit awkward reeling in again - need to experiment with positioning of hosereel so handle is easy to reach and turn.
That'll be £5,000 consultant fee, thank you

Cheers,
Ian