Home natural gas adsorbtion based units did exist (and still do, to some extent). They work just as well as a compressor based air conditioner, but BTU/BTU they aren't near as efficient. They only make sense if there is a very large price difference between natural gas and electricity.
On an RV, the gasoline needed to drive a generator to power a compressor based A/C will cost a lot less than the propane needed to fire an adsorbtion A/C. Look at it this way, as soon as you hook up shore power, what doe the refer do? It switches to electricity. There is a reason.
Now, that's not to say I don't see an advantage to having a propane, or dual-fueled generator on board...