Internet Service
Pretty much answered above, but to reiterate:
1) Depends on the RV park, whether it is available and how much. I suspect one of those big books listing 'all' the campgrounds in an area may say, so choose the ones you like and then call ahead to insure they have it available, and the cost.
2)If you get Verizon, and you compute at night and over the weekends, you might be able to get away for as little as $40 a month. It should work in about 95% of the country, best in the Eastern half of the US, almost as good in the Southwest, and mediocre in the Northwest. Depending on your phone, you may have to buy an interface card for your PC, perhaps $50 to $200. Don't know about any other provider
3) Don't have a clue. You would need a WIFI enabled computer and then check with the RV parks you were thinking about staying at. I'm thinking it might be 'free' or reasonably priced, but perhaps fairly rare at this point in time.
4) Last I heard, it was $100 a month after $2500 or more installation. Of course, bound to come down over time.