WiFi in some RV parks has probably improved a bit over the past few years, but most are still not set up to allow video streaming or online gaming. That requires a large expenditure that most parks can't justify. So most of the time, you'll run into either speed caps, or data transfer limits, or timeouts, or some combination of the above. Or you may be able to purchase upgraded service for an additional charge.
As an example of what can be done, Tiger Run Resort in Breckenridge, CO provides 7 Mb/sec downloads with a 20 Gigabyte per day data limit. Most video streaming takes about 1 Gigabyte per hour, so that allows plenty of video or gaming. But that installation of equipment was about $90K and the ongoing cost is about $40K/year. It's expensive.
I've run into other parks where you can stream video, but every 2 hours, the system kicks you off. I've also seen many parks where the WiFi barely worked at all. And others where it worked well while the park was almost empty, but service degraded as the park filled up. You're also likely to run into authentication limits, where the park only permits one device to connect at a time.
Service is limited by 1) how large a pipe to the internet the park gets, 2) the quality of their links and access points to distribute the WiFi signal, 3) how crowded the park is, 4) your distance from the access point.
I use a WISP Router that connects to the park internet, on 5 GHz when possible (to minimize the problems of a crowded park). The router authenticates to the park. All of my phones, tablets, Kindles, Directv, Roku, printer, and laptops connect to the router and share the WiFi through the router. That arrangement lets me connect 10 devices at the same time even where the park only allows one device to authenticate.
Some people get routers that have removable antennas and they install high gain external antennas.
But coming back around to where I started, if you expect to stream video on your Roku via campground WiFi, most of the time you'll be disappointed, no matter what equipment you purchase.