I don't think you will see any drop in mpg with the truck ac on.
Unless you are driving a little 25+ mpg car that only has 75hp, you won't notice a difference. And on the 25+mpg car u will lose mpg with windows open-vs ac on. I've tested it on my 100+mpg daily commutes.
If you think the genny will save you $, go for it. But if your driving a 10mpg gasser, I doubt you'll even notice any change in fuel consumption- and less if its diesel.
MillerTime- Future Sabre Handler!!