240' is a long way for an omni-directional wireless router setup. If you have a Fry's or similar "geek" store nearby, go in and talk about a directional wireless lan extender. If you are a true geek, you can cobble one together with a Pringles can (
http://en.wikipedia.org/wiki/Cantenna and
http://www.oreillynet.com/cs/weblog/view/wlg/448). Ideally, you would get a couple of wireless bridges and link them with a directional antenna (e.g., yagi). With so much distance, I'd recommend a commercial, rather than a roll-your-own, solution.
I have a friend who set up something similar for a large RV park that he manages. I can ping him to see what he eventually did, it you want.