Before I moved to a Van I had a 50 Gall loft tank (baffled myself) in the back of a Montego Estate, put the battery and pump inside and fed a line to the outside for the water.
Fitted the Hose reel to a bike carrier on the outside with the pole resting nicely on my ladders on the roof rack.
The Vehicle started the day quiet low down but soon started to rise as I used it, never bother to check on the legality, but it never gave me any probs and I was tailed several times by Mr plod
the picture gives you some idea this was mark two before I got fed up with taking the trolley of the back and decided to put tank inside which was mark 8