Get one of the 5lt pressure sprayers, available from garden centres and the like, then mix up a shampoo/water solution or use a dedicated pre wash treatment and spray the whole car, followed by a dwell period (depending on what product is used) and rinse, then onto your normal wash procedure.
I use this method and sprayer all the time and well worth the small time/cost needed. I would guess given that water supply is a prob this would be even better for you.