Unfortunately, WiFi signals originate from the WiFi router and there is not a lot you can do to suck more band width or radio signal into your receiver.
A signal booster has to be put on the originating router to increase the power of the emitter. So, unless you can get the campground owner to invest in better equipment for boosting their signal, you're out of luck.
Of course, that said... You could try one of these...
an "Extended Long Range WiFi Receiver System
It may help, but, again, it is still contingent on the originating signal.