This was my shot at the smallest trailer to carry a working shower:
Finding the space for a shower isn't that hard, but then you have to have a large water tank, a hot water system and a grey water tank. None of this lot is either small or light, so you have to build a fairly big trailer.
Len is building something very much like this right now - in
his Yahoo album, scroll down to 'Shasta 1' to 'Shasta 6'.
Andrew