I see your point about killing gas mileage and the cost. My 'Nest only cost me $400, doesn't kill the gas milage. Really good tents aren't exactly cheap. Sure is nice sleeping on the foam pad, I never had any luck with air mattresses, I was carrying a small futon mattress, but, it took up alot of room and heavy. My bad back can't take those small self inflating sleep pads.
I personally wouldn't own a RTT (they aren't practical enough for me), only what I have already or a hardtop camper version like I posted earily.
For some reason we feel safer off the ground, away from the critters, unlike the RTT, I can stand up in my Wildernest, cook, change clothes, etc. The dog sleeps on the bed of the truck, no worries when she has playing in the creek,
From an environmental veiw, it has a lower impact than a tent, less ground contact area. So, I don't know what kind of environmentalist I am since I ride dirt bikes and drive a 4WD truck...
My VW camper sets up in less than 30 seconds, now only if I could get something like that for the Tacoma, that would be the cat's meow...