As mentioned above, you usually have two options: Campground wifi, or mobile hotspot.
Regarding campground wifi - it usually sucks. Sure, you get a great strong signal and connection to the router, but the campground usually has crummy dog slow service. On top of that, you're usually also fighting everyone else for a piece of that slow connection, and it slows down even further. Because of this, we almost always rely on our mobile hot spots provided by our phone.
Verizon, in my experience, has the broadest coverage. I can get a 3G signal from them most anywhere I go. I'm getting LTE high speed connections at some places now as well!
I cover both of my bases - I'll try the campground wifi and see if it's acceptable before using my expensive cellular data plan. Make sure you have a high enough data plan that you won't go over, because the cell providers love to charge you an exorbitant amount if you go over. Verizon lets me increase my plan through my iPhone via their app if I need to do so, on the fly. We've got 3GB by default, which usually covers us for a 4-5 day trip each month. Two week camping trip? 6GB+. (My wife uses the internet a LOT).
As Bob mentions, another failing of campground wifi is weak signal. Here's the antenna booster to get: Amazon.com: C. Crane US3 Super USB Wifi Antenna 3: Electronics
Yes, there will be folks that tell you to get a router, mod it with software that can extend the campground wifi, and put a big antenna outside. I'm an IT guy, and this method sucked. It was not very productive, and cost a bundle. The little Crane antenna I linked works great. I plug it into my Mac via the USB port, run the software provided, and then share that data coming in through my wifi to our other computers and mobile devices. (Yes, we're nerds, and travel with a lot of gear).
I'll be happy to answer further questions!