I can tell you the fix from the Redwood and Montana factory was simply a cut in the wall below the slide with an H-channel trim inserted. Those that had them from the factory have not had issues. If it's frame flex, so what as long as you accommodate for the flex.
Now there is a big difference between frame flex and frame fail because of poor welds - different subject.
I know all you box frame lovers are compassionate about your box frames, to the point of it being a cult, but that isn't the only way to build a coach.