Welcome to truckconversion.net! I'm glad you're looking for a truck conversion. I don't know anything about the different construction methods so no help there. I think if you're looking at used units you would just want to make sure it's held up well. Take it for a drive and see how tight everything is. I don't know if a truck conversion is any less prone to problems than a "regular" motorhome tho? But, when they do break down, if it's a truck problem, you can pull into any truck repair place rather than a r.v. shop. Truck shops are cheaper and more plentiful. All of the other systems in a truck conversion are pretty much the same as any other motorhome so just as likely to break down.
Okay, Geofkaye will post soon about construction of the box methods and such.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio