WISP is just a mode of the router and many of today's routers can do that. Basically it just means you one wireless router acts as a repeater for another.
Please tell us more about how you are trying to connect inside? Do you have a repeater inside the RV now? Or is the Roku trying to connect direct to the campground wifi? If so, your problem may simply be a weak signal inside the RV, a common problem in campgrounds. The shell of the RV does a good job in inhibiting the signal, which is often weak to begin with. If that's it, then yes some kind of wireless repeater will probably help.
Does the Roku stream ok when outside? Few campground internet systems have enough bandwidth the stream effectively, at least not all the time. If somebody else starts making heavy use of the internet, your Roku may slow to the point where it is unusable (pixelation or drop-out). And if yours is working, probably some other camper is having problems...