i have a 1977 toy chinook, it used to have ac, it looks to be a super easy installation, you dont even need to dismantle anything under the bonnet,. it bolts to the left, near the oil filter.
as ac goes, i prefer not. you would need to attatch an entirely new belt, i dont like. even though it will only put little fractions of strain on the engine, and only when you have the ac in an on position. better mpg? debatable. working your old engine harder on a hot day, not cool.
myself, i have been thinking of installing a simple, inexpensive, vent/fan in the roof. unless you need a cool breeze on a hot day, go for it.
although, you will need to purchase the toy first.
have the only camper in town that can peelout. just keep the old tires on.