If you have a 14"x14" vent it can be installed there and a small unit doesn't weight all that much, 90 to 120 pounds for a small unit.You'll just have to figure out how to power it. Dometic AC Coleman AC Polar Cub AC
You will have to figure out a way to install it on the roof without walking, kneeling, etc. on the roof. Most are too thin to hold ones weight. You will have to reinforce the area where the AC will go, and of course get it wired up. You will need something around 11,000 btu's or more, and only expect about 10 to 15 degrees max of cooling.
I would also be concerned about the noise the unit makes in a small metal van. I just did this on our American Cruiser class "B", a bigger and likely more sound muffling than what you are talking about, and the sound when it operates is loud, but OK with us, just beware. I agree you will have to reinforce the roof in the area of the install, the good news is that as stated above any standard RV vent hole should work. Snaking a 12 gauge wire to power it may be your biggest challenge.
My Roadtrek wrote: You will have to figure out a way to install it on the roof without walking, kneeling, etc. on the roof. Most are too thin to hold ones weight. You will have to reinforce the area where the AC will go, and of course get it wired up. You will need something around 11,000 btu's or more, and only expect about 10 to 15 degrees max of cooling.