Changing from 14" to 15" wheels should not make it necessary to install air shocks unless you've gone to a really wide rim and tire, say over 8" or so.
The main reason air shocks leak down is the joint fittings, the schraeder valve, and that crappy plastic pipe that comes with them. If that pipe is anywhere near a pinch point, it will be pinched and ...bingo, slow leak (in your case, major leak, 'cause a slow leak usually takes a week or more to go down), or if it is installed anywhere near the exhaust, melt down, leak city.
I would bet that your Camino might just need a new set of rear coil springs too. They are available very reasonably from any O'Reilly's or CarQuest or NAPA store. They are fairly easy to install, but can be dangerous, act accordingly. I installed a new set of springs on the 76 Camino I had and it raise the tail almost 6 inches, yes, half a foot!! The new front springs changed the whole picture there too, up almost 4 inches. After the first coupe of weeks driving they settled in at about 5" in the rear and 4" in the front. But what a difference in ride and handling....WOW! The car looked better and drove and rode better too. All this for less than $135 in parts.