How can I improve my wireless connection with my iPhone when at RV campground??

Do you mean your cellular connection? Or your WiFi connection? Short of building your own cell tower, there's not much you can do if there isn't good cell coverage where you are. You can call your carrier and let them know that the signal is weak there. If enough people call, they'll probably add the area to their list of places to eventually put up a new cell.

If the issue is WiFi, that's something you'd need to take up with whoever is in charge of the WiFi network in question. If the park is providing WiFi, you could discuss with them the feasibility of them extending the network.