Hi....they start with a huge table that is perfectly flat and lay out the outer skin (if fiberglas) add the aluminum structure or frame, insert the insulation and add the inner wall with heat activated adhesive then cover the assembly and then apply a vacuum and heat, the vacuum pulls everything together and produces the flatest and strongest wall, floor or roof assembly that can be made. That's a bit abbreviated, but easiest to understand. If you ever get a chance, visit Jayco's factory, I recommend it.
Mark, Debie & Belle the wonder dog
2013 Prime Time Crusader 36 ft 5er
2008 Chevy Silverado 2500HD crew cab, 6.0L