The internet is full of these pictures but you miss the point.
40 years ago the manufacturer flat out said on paper.....
"Your pickup is not recommended for carrying slide in campers."
there must be a reason this is happening....
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Oh, I got the point. It's pretty obvious the legal eagles in the OEMs are playing CYA in so many ways. But as you noted it's been going on for years. I've got a pic of a Label in a '91 K5 that a buddy has that he bought with a Four Wheel Camper on it like mine.
Yep says the same thing. Ironically it was the K5 platform that just over a decade earlier GM was selling *gasp* a slide-in camper in a "factory" package as the Blazer Chalet/Jimmy Casa Grande. The body didn't change in the underpinning from 1977 to 1991, the frame didn't change either. So what caused the change in outlook at GM besides the potential liability?
What makes it even more ironic is how much larger/heavier a Chalet is vs a FWC unit on a Blazer. Anytime you extend weight beyond the rear axle it has an effect to cantilever on the frame with the rear axle as the pivot point. The further it is from the axle centerline the worse it gets. The examples you pulled from the web only prove that point. All you showed were ultra-short short boxes where the majority of the camper is past the axle centerline. In those trucks there isn't any solid way to carry a camper and not have that happen, hence the disclaimer is pretty accurate. Though it doesn't make sense that they also put that disclaimer on larger/heavier trucks that have longer beds that would properly carry the weight of a camper within it.
It's not that GM and Ford don't know how to build a truck specific for camper duty. They did once upon a time. During the 68-72 generation of trucks they produced the "Longhorn" version that added 6 inches to the wheelbase and the bed of the truck. This allowed for the CG of the camper to be in front of the rear axle for a better weight bias.
Ford's version was the Super Camper Special which didn't increase the bed length but moved the rear axle back to a whopping 140" wheelbase.
Between rising fuel costs and other changes in the automotive world, these giants just didn't pan out as volume sellers. Which sucks because I bet they handled big campers so much better than ones with a shorter wheelbase. Could you imagine a new GM, Superduty or Ram built with this kind of radical wheelbase change to handle a large camper better?