As was noted in the past, the term "frame flex" should have been termed "wall frame flex" so that there is no confusion with the main steel trailer frame...which also had flex issues in the past. If the wall framing was tig welded prior to assembly, there would be no issues. Spotty tack welding by mig is not desirable, but time is money.
2012 F350 6.7 L dually, 2013 3800RE with 6 pt leveling, Sumitomo 17.5" load range h tires, Samsung 18 cu ft residential fridge, 8k Morryde I.S. with disc brakes. Full timing since 2012.