Your best bet would be to run the generator right next to the Airstream, on the opposite side of the wedding party, set it for maximum cool, and then have it turned off right before the ceremony. The trailer will stay cool for about 10 minutes in direct sunlight.
After the ceremony, you could turn it on again, but any length of extension cord will again cause severe problems for an AC unit. Any airstream owner would be wise to have you sign a contract indicating you would be liable for any damages (drunken guests using the facilites, burned up AC....)
If you were to provide a range of what you were willing to pay, that might get a few more takers.....