I was thinking that a standard slot drain might work but wonder if they are too deep for your use? Whilst the surface would be flat the drain itself would need ot "hang" below. If this of interest, if you google "slot drains" there are a variety available.
This company seem to have a variety of options but look expensive :(
http://www.slimlinedrains.co.uk/applications/bathroom-drainage
What is below the floor? Is there an "underfloor space" or chassis etc. immediately below?
Depending on this, the other thought I had was whether you could have a covered "gulley" (e.g. a bit of guttering covered with "floor")? When in normal use the floor is fine. When your wife goes over it into the shower the relevant board can be lifted (maybe related to the tilt of the caravan) and the shower water goes down the "gulley". Very cheap option :)