There are a number of things to be worried about:
1) if there is no management password or the factory default password is still being used for the access point/router/extender itself, than your "friendly neighbor" would just login to the access point and look at the WIFI password. I would say that is the most likely scenario, your provider probably left them as default or use a generic one for all their customers. I would start by logging into the web interface of both the router and the extender and change the password to access these devices from the web (the management password).
2) if WPS is enabled on the access point it is very easy to hack the 4 digit code, it only takes a few minutes to get the code. WPS is way to insecure, never have it enabled, find a way to disable it all the time.
3) if you are using WEP for encryption, the pre shared key (password) can be hacked in minutes as well. If this is the case change your encryption to WPA (preferably WPA2 with AES only).
4) even with WPA it is possible to hack it in some circumstances but it would require someone with a high level of skill and access to some high end hardware. No recommendation here, I would not worry about it, just be aware that nothing is hack proof.
And yes it is possible in a campground as well, but it depends on your security, and how determined someone is. If you have a strong management password for the router, disable WPS, use WPA2 with AES, and use a strong WIFI password you should be safe from all but the more skilled hackers or really bored high school kids.