The interior carpet was one of the first things to go when I got the 4runner about 12 years ago. The interior's all Herculined. The only thing I took out was the rear seatbacks. The rear seat bottoms are still in place.
The camper's wedged in there pretty tightly at the back. I had to trim some of the metal seam around the tailgate to get it to fit width-wise. At the front, there's one of the big (about 3") cargo straps that runs through the front bottom of the camper floor and is tensioned into the front seatbelt anchors.
Someday I might bolt the camper down, since it stays in there all the time (I don't even have camper jacks). It's been working pretty well the way it is though.