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.