15 inches makes a big difference in weight and wind drag. When the top is up and slide out, the interior is very spacious for a truck camper. So far I've seen 17 mpg @ 70 mph fully loaded with an engine that's not broken in yet.
Sorry to perplex. By the way, what is your cup of tea?
It's a cool camper, my post comes off a bit harsh. You had a dream and made it come true.
My cup of tea, in a nutshell, would be the XP camper. I just can't afford one of those and also don't like how long the wait is to order one.