I think the question asked by the OP has been fully answered. His finding is that even if parks advertize WiFi that the WiFi is poor at best and everyone agrees. We can debate whether or not we should expect broadband quality WiFi at our campsites or whether a person should want Internet access at all if we want, but the fact is that a person is naive if they expect to have reliable and fast campground provided internet everywhere they camp.
That leaves us with the original concern: what's the best way to provide your own internet access?
My thinking is that unless one is fortunate enough to have a grandfathered unlimited Verizon data plan and can use their cell phone as a hotspot that the next best thing is 20 Gigs of data for $70 from Millinicom. That's the solution suggested by several and I agree that it's a good one for fulltimers.