LTX M/S2 is newer version of M/S, and the M/S2 is designed as a LRR (low rolling resistance), fuel-saving tire. Tread design is essentially the same, but tread compound and construction is advertised to be slightly different. Considering fuel consumption of most TVs, a LRR tire will probably not save a significant amount of fuel, compared to just driving slower.
Neither tire will probably wear out on an Airstream, unless you are a road warrior; so I would personally buy the cheaper tire. However, insure that the maximum inflation pressure printed on the sidewall is 80 psi, which indicates it is an LT tire. If the sidewall indicates 44 or 50 psi max, it is a passenger car tire.
If you have 16-inch wheels, for the extra cost, the LT tire will have heavier construction, including a third steel belt under the tread, and a 7-800 pound higher load rating.