A couple of ASSUMPTIONs that should be addressed here first.
Class As have, on the whole, pretty awful crash safety performance. They're exempt from the rules that govern passenger vehicles. In many cases the roof will not support the weight of the vehicle, and the requirements for attachment of seats and seatbelts to the floor and frame are not rigorous.
A combination of industry lobbying and the small number passenger-miles in RVs compared to cars has meant that this problem has never received much regulatory scrutiny.
So, is a truck camper as safe as a class A? Well, perhaps, but maybe that doesn't mean much.
As for laws, be careful. In Minnesota there is no law against having passengers in a truck camper or travel trailer. But a creative cop could write a ticket for seat belt compliance or careless or reckless driving, and some do.
Lou Schneider: I think you'll find that all the windows in any camper, whether a truck camper, TT, or MH, are safety glass, by law. Also, a PROPERLY tied down camper using Torklifts or a comparable tiedown that attaches to the frame, will not separate from the truck during a roll over or collision, any more than the body of a class C would separate from the frame.
The real problems are different.
The environment of a truck camper being separate from the cab, the driver may miss warning signs that something is amiss. Kids doing stupid kid stuff, exhaust fumes and CO accumulating, that kind of thing. So there has to be a trustworthy person back there and a means of clear communication, like a sliding window -- something not always present.
And the roof of TCs isn't built any better than the roof of class C or class A rigs making the risk of injury during a collision or rollover (especially) very real.
The lack of seatbelts is there but a trip to the junkyard and a couple of lag bolts can fix that for you.
I like my Suburban with 3 rows of seats and consider it a safer way for family members to ride along than the various other options out there. One of the many considerations that gets ignored in the MH vs TT debates.