I have ridden in fully equipped truck campers. It isn't bad at all. No more bouncy than most older motorhomes.
Most all of them have windows
that open into the cab of the truck.
Byron is right, I don't think this is about TRAILERS, but pickup campers.
Some states allow you to ride in 5th wheels too, but I believe there has to be an intercom between the trailer and cab.
Conflicting laws.. seems I recall Washington used to require working windshield wipers on all street legal cars and trucks, but they did not require windshields themselves. Or was that an Oregon burp?
It's been fixed since, but I remember it being a big joke on the local news for a few days.