Mostly you are correct, a box on wheels but there are minute differences in quality of material, chassis, house structure materials, higher tech this and that, etc.
I agree it doesn't make much difference in size. Once you go above about 36 feet having quad slides gives you more floor space in the middle but along the sides only the innovations of kitchen cabinets that slide out and couches that extend for more seating...it is what it is. If you go with a 45' with 2 bathrooms, more floor space or add bunks...yes a difference but for our lifestyle we just would not benefit from it. We are comfortable with our size rig and don't really need more room anywhere, just some rearranging for convenience.