Well, my understanding is that for a window unit to work right, it has to be mounted through the wall, with the inside part inside the room and the outside part outside. You can't "pipe" the cold air. It's not meant to push air through a ductwork, and needs to circulate the air in the room. The intake and exhaust are both on the front. Cold air is blown straight out into the middle of the room, and warm air is pulled in from above off the wall. Inside a duct, it would just circulate the cold air and shut off after a few minutes. cold air would not make it into the vehicle.
To that end you would have to be able to hang a window AC unit on the Suburban somehow, probably on the back so you don't have it hanging out the side as you go down the road, and/or don't have to remove it every time you decide to pull up stakes and go somewhere different.
A rooftop unit is certainly doable, as all it takes is a 14" square hole in the roof and maybe a little bracing.
In either case though, you're cutting up your Suburban and making it unsellable down the road when that eventually happens. Hopefully you will love it so much that you'll keep it until it has depreciated to the point where it doesn't matter, but it might be something you want to consider.
Really, ANYTHING is doable. I know taller people sleeping in shorter trucks and loving every minute of it.
