greygoos - That is a good looking truck. When I was designing my camper i was very, very hesitant to build it at permanent standing height because of the hit I thought I would take on mileage. Over the last 600 miles of mostly highway (55-70 mph), and some around town driving I've been getting 21.8 mpg. My truck has an epa estimated 21 mpg on the highway..
I owe this to the fact that I drive like a TOTAL GRANDMA. other than freeway on ramps, I never have the gas pedal more than 1/4 depressed. If you don't plan on having the camper on 24/7, my advice is GO BIG and dial back your speed and acceleration to save fuel.
In my opinion, folding walls/popup canvas designs are MUCH more complicated, expensive, prone to leaking and more time consuming to build.
That being said, I would love to see any popup design you can come up with!