Dave, I was just googling for info on my black water tank and bumped into a page about the photographer on the Capetown to Cairo trip. Then I logged in here for the first time in months and saw your question. Here is the relevant part of the article about the photographer:
>He doesn't recall being daunted by the assignment. What did make him a little nervous was the idea of driving across the continent. "But the Airstream people took care of that," he says. "They built--what would you call it today? A recreational vehicle?--equipped with everything I could need. It had all sorts of gas tanks so you could go miles and miles without stopping. They even built a special compartment around the water tank where I could store my film and keep it cool."
I think your mystery vehicle is none other than the photographer's custome RV!
I don't recall if we are allowed to post urls here, but hopefully this will work: