We've been to campgrounds with good and really good WiFi, but they seem to be few and far between. Fortunately, our seasonal campground has very good WiFi. I have a 4 GB/month plan on Verizon with my smartphone that is used as a Hot Spot when campground WiFi is not available or isn't reliable. We don't usually stream video when camping, even seasonally. The WiFi at our seasonal site has enough bandwidth and is reliable enough to stream video, but we usually don't. We use about 6 GB/Month without streaming.
Streaming video at 720p resolution will use about 4GB of data for a 13 episode, 30 minute show. Watching the occasional show on Netflix might be okay, but if you try to use it often, most likely you will not be satisfied. We've been to campgrounds where they actively monitor usage. At parks like this, you would probably be blocked from accessing their WiFi after watching a few shows.
We don't expect good WiFi when we are camping and we don't expect to stream video reliably. Someone mentioned getting a Cell Provider that offers free video streaming, I think AT&T and T-Mobile offer it. Going with Unlimited Data from say AT&T or Verizon will only get you so far. Both companies claim they throttle speeds after using 22 GB/month.
-Michael