The new Michelin M/S2 indeed does have a different sidewall construction.
Just read right on the side of the tires (fine print) the makeup of the M/S2's sidwall material, then compare this with what it says/said on the sidewalls of the old M/S tires.
Also, the new M/S2 tires now carry a 70,000 mile warranty, instead of the M/S tire's 60,000 mile warranty. Even though you probably won't accrue anywhere near this kind of mileage on your RV's tires, what this higher warranty really gives you is that for any given mileage on your new M/S2 tires they will have more tread depth left - which means you will have better road debris puncture resistance.