Phil,

You're not serious are you???

I have a small home a/c in my house and it draws about 15 amps at
120v.  Run the numbers... volts times amps equals watts.  1800 watts
at 120v.  Remember TANSTAAFL? (There ain't no such thing as a free
lunch)  That means that to get the equivalent energy at 12v we need
only multiply by ten.  We now need 18,000 watts at 12v. (The biggest
Vanagon alternator puts out 90 watts) Or, working the equation
backwards, 150 amps at 12v. That's a bunch my friend. 'Bout the same
as your starter motor draws.  Unhook the coil wire and crank that
thing until the motor won't turn over any more. Time it.  That's about
twice as long as your battery will run the inverter to power this a/c
unit.

It's do-able, but you ain't gonna like the price!

Run the home a/c unit plugged in at the campground.  Roll down the
windows when you're driving. (Or add an automotive a/c unit.)

Jim