The combat photographs in the days of yore shipped their film back to the publications lab for processing and printing. Remember the incident with Capa's film from the D-Day invasion? Today's combat photographers use d******.
I don't want to rain on the parade, but I can't really see any practical need for a travelling darkroom unless you are permanently on the road. Years ago, I used to sleep in tents when traveling and camping and I shot with a 4x5. I carried a changing bag to load and unload holders. That worked okay but it did not involve the complex set of problems you will need to solve.
I have a 28 ft travel trailer I use for weekend and longer trips and I considered putting in a darkroom or, at least, taking along the film developing equipment and supplies. The problem always came down to two things, space and light. Virtually all the RV's, whether monster motorhome/buses or large fifthwheel trailers are designed for actual living within a minimum of space. Storage of items is a big problem on smaller RV's and the openness of designs on larger RV's makes for a lot of outdoor light to be let inside.
My trailer has a lot more space than your VW camper but it becomes awfully small after a week or two with myself, my wife and our dog living in it. The items we need for normal activities of daily living take up a lot of the space. My camera equipment--and I take lots of equipment when I use the trailer--takes up a lot of room. All this necessary stuff needs space for storage when not in actual use and there's just not a lot of it. After some contemplation, I decided to abandon the idea of a traveling darkroom. I can do a better job at home without the hassle. My solution was Ziplock bags. Fill them up with exposed film and wait until later to develop them.
I think Brett Weston had a darkroom van that he used for loading his 11x14" holders and processing film. You might see if you can find any info on it.
Permawash will cut your wash times substantially, and you can use 5-7 changes of water in place of running water. I think during a water shortage Ansel Adams determined (using a residual hypo test, I'm assuming) that 7 changes was good enough for fiber based prints. The setup was 7 trays in a long sink (stacking would be the obvious way to save space), and the system involved periodically dumping tray 1, moving the others up, and filling it with fresh water and moving it to position 7.
If you wanted to try to print in a van, you could look into a slot processor.
I'm thinking that a UHaul type trailer with a used Fuji/Noritsu 1hr processing lab should work out. Use the back of the vw for storage and napping while the machine does its thing. This way you can also set up a processing biz while at each locale to help cover the costs of your own developing...
Just kidding...I think...
Fixer scented Glade; for those that just can't leave the darkroom.