Brad- IIHITDOA, I'd put the membrane alongside the frame rail on the driver's side, probably just hung from the ceiling. This area is otherwise dead for storage volume, and easy to work on after you remove enough of your junk to work overhead on your back there.
For the RO water connection to your storage tank, you can follow the pex inlet pipe from the top of the tank rearward to find an easily accessed space to splice a tee.
The Vag manual has a good description of where/how to cut in the diverter setup.
You will probably want two good quality check valves, one on either side of the tank-fill/city-water valve; this will keep the on board pressure pump from pressing water back thru the internal workings of the valve so that the pump runs a small amount every few minutes.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project