Actually, with the tank on the roof, I may not even need a pump....
Might rig up something for a shower though....
Got a couple of outdoor shower kits, one from Maplins that runs off the cigar lighter or D cell batteries the other from Lidl, a garden shower type that you connect a hose to, with a small drill powered pump it goes really well.
LS
http://www.maplin.co.uk/Search.aspx?criteria=car shower&source=15&SD=Y