Some campers are usable with the slides in, many are not. You need to go look at the camper in question to see if it is workable for you.
As far as crawling through the window, you'd have to be pretty small. The sliding rear windows on late-model trucks don't leave you much room to wiggle through.
You didn't specify whether or not people would ride in the camper while underway, but between you and me it just isn't a good idea in this day and age. There are a lot of people who will say that they rode in the camper through the '60s and '70s and everything worked out just fine. Of course it was a different time then. There wasn't nearly as much traffic, or nearly as many distractions, or nearly as many unskilled drivers.
Back in the '60s and '70s there wasn't much difference between riding in the camper and riding in the truck because seatbelts weren't mandatory, airbags had not been invented yet, there were no side impact beams in the doors, or rollover protection in the roof, or crumple zones. You were pretty much boned in an accident no matter where you rode. Trucks made in the last 15 years or so have all sorts of safety systems to protect you in a crash, but campers have not advanced in that respect.
