You can buy a sort or two-in-one thing: a sink tap arrangement with hot and cold controls either side, and in the middle a shower head as the tap, on a long length (maybe 1.5 metres?) of flexi hose that sits inside the sink unit.
You can then just pull up the showerhead and attach to a mount on the wall when you want to use it as a shower rather than a tap.

Maybe worth considering?

This sort of thing:

http://www.ebay.co.uk/itm/CARAVAN-MO...item3ceba4bbac