I saw an article in Pop Photo, must have been about 1970, where the guy had a whole darkroom in his VW. I don't remember why. Little enlarger, trays, the whole bit.
Seems like Rodinal or HC110 would be good developers. Easy. Won't go bad. You need something that will work at different temperatures. And definitely some way to dry negs, because you will have lots of dirt. Tight box to store negs, with desiccant if you go somewhere humid.
Happy Trails and
Watch for Loose Gravel