OT:
Sod, I've posted a few links regarding this.
If you want the BEST batteries money can buy, get the odyssey pc2150.
The way you measure "storage" of the battery is amp-hours.
http://www.odysseyfactory.com/specs.htm
pc2150 is the group 31.
It can sustain 10 amps for 10 hours before voltage drops to 10. You can still charge in the summer at 10 volts, but it can be tough on your equipment.
X4 = 40 amps for 10 hours straight. 40 amps X 12 volts = 480 watts for 10 hours straight.
You do *NOT* want a cheap a/c unit. you want a SPLIT unit. They're far more energy efficient. Here's a good one:
http://www.ajmadison.com/cgi-bin/ajmadison/09KLS71.html
It has a SEER of 16, far better then any window unit. It will also seal better.
Honestly, 5000 BTU's can definitely work IF you have a well insulated truck.
In my volvo 770, I used to park in the sun during the day in minneapolis, and I never got too warm with a 5000 BTU a/c.
My dad regularly runs an a/c unit, but from his batteries. It's hard on batteries, you need an AGM battery! Regular batteries WILL NOT WORK.