I spent a few minutes trying to connect to the Internet last night and I can now officially give my recommendation to the Hawking Hi-Gain USB Wireless-G Dish Adapter. Neither of our laptops would connect to either the park WIFI or the Pollo Feliz restaurant on the corner. When I add the Hawking dish, place it on the dashboard and point it at the restaurant, I get a 44% signal strength and can connect easily! I am very happy with this unit!
EDIT: A couple of readers have asked where I got the Hawking dish. I was in Yuma when I decided to pick one up and an RV'er across the road said she had one that she never used and would be willing to let go. We had quite a battle over the price as I wanted to pay a fair price and she wanted to give it to me for free. We finally came to an agreement but I still paid far too little. They are available from Best Buy, Newegg and many other places in the $80 - $90 range.
The only negative comments about them is their difficulty to work with Windows Vista. I have Windows XP and have no problem.