I have heard-tell of RV seatbelts, for seats other than the up front driver and passenger, being bolted to plywood, and other less-than-strong added parts of the vehicle. The Roadtrek RVs are not put together at a big vehicle factory- they strip out a vehicle (like the Sprinter) and then build in the RV stuff. I'd trust the front seats, but there's no real reason the rear seats would have truly reliable seatbelts, and many seatbelt laws exempt motorhomes, or exempt passengers not in the front seats.
Washington's law is explained like this:
"Do persons in a camper or motor home have to wear safety belts?
"Persons in the driving compartment must use proper restraints. Persons in the living area should use restraints if there are restraints available. "
So, uh. I don't know. I'm optimistic, for some reason, about that Roadtrek.