To attach an external antenna to the Westell 7500 would require modifying the unit which will void warranty of the device. It has internal antennas only. You can get an external high gain wireless card for the laptop but that will only do so much, and access points can be hung off of the Westell 7500 and located somewhere to help improve service in dead spots. It's honestly up to the park to beef up their Wi-Fi service for the campers if this is their service by putting up outdoor high-gain WISP-grade antennas and APs that are capable of serving up the park with a solid signal.
I'd point out a really nice antenna/radio combination to use that really works well in RV parks with lots of metal but I'm having some issues finding it. Either way though, I'd say the best choice would be to ask the RV Park to beef up their Wi-Fi. If they look at the right equipment it should be fairly inexpensive.
It may also be a consideration to see if you can get your own service too from the park. If they allow you to order Telephone service from Verizon, see if you can also get DSL put on that line. It's not free but it is an option and a good choice if available.
