Here are your options and a bit about them
Park Wi-Fi... This is for the enjoyment of the park guests, NOT the EMPLOYMENT of them, please do not use park wi-fi for commercial use.. Generally limited bandwidth and this can lead to frustration, also not all parks offer it.
HughesNet or other 2-way Sat provider.. Expensive to set up, works nearly everywhere (Generally if it does not work a simple change of site within the CG is enough to get it working) basic Hughesnet is low speed and limited bandwidth, Gen-4 is not compatible with RV life (but is faster) Exceed is like Hughes Gen-4 (Basicaly identical).
Cellular: NO cellular company has 100% coverage, (Verizon has the best coverage but... I have been to places they are not).
There are generally bandwidth limits, streaming videos can slam up against the wall on those.. There are overage charges on most plans.
That said T-Mobile offers unlimited plan,, the catch is only the first so much (you decide) is "high Speed" (Streaming video quality), the rest is throttled (I get 7 Gig) and they just started a data-stash (10 Gig) which is basically Rollover bytes,, If I only use say 3 Gig this month (I am mostly on park/other Wi-Fi) the remaing 4 Gig rolls over,, Stashed for up to a year. You use your Stash first so you roll over fresh bytes nearly every month (Less you max out).
And finally there is McDonalds et-al, but you already know about them
I've done the library trip a few times too.