I'm handy. I have built a house with 50% recycled lumber. I've worked on several rvs. I don't know that I'm up for making a custom pop-out for a 30-year-old motorhome. Frankly, that sounds like a huge undertaking, with considerable welding and engineering, just the sort of project that takes three times as much money and time as the initial budget. It would be cool to make it look like a bellows camera though! And of course with some mural paper and a .33x achromat you get mission creep- but a cool story, and having a story sells art.
I do like the idea of a dedicated area, in the Toyota Dolphin 21' I used the bathroom which blacked-out easily with just velcro curtains, but it was quite small. In a Winnebago 30', I just used the kitchen with plastic everywhere but that was a cleaning bother and I couldn't make it dark enough to tray develop film. Perhaps if I got a model with a large rear bedroom that would be best. Many models have the bathroom sharing that wall which could make plumbing easier. I prefer a smaller vehicle, but I need what I need if it comes to that. I don't need a bedroom for myself, an overhead sleeper would be fine and the big-puppy could have the sofa. I'd been envisioning a smaller vehicle though.