What are the etiquettes to charge at a campground or RV park, do I need a prior reservation? What if it's an emergency and it's outside business hours, can I just show up and hook up the car?
Good questions, and the answer is basically: it depends on each place. Some are independent, some are part of a bigger chain, and management at each place is kind of different. I can give you a few examples to show how some of it varies.
The ones listed on Plugshare are generally places where some intrepid EV driver has blazed the trail ahead of you so there has been some discussion of policy or agreement worked out. When I first got my Tesla in March of 2014, I went on a trip within a few weeks along an interstate that didn't have any Superchargers yet. I called Anderson Campground RV park, just east of Twin Falls Idaho (not on Plugshare yet at that time) and talked with them about my travel plan. I said that I was planning to go ahead and spend the night there and car camp, so I offered to just rent an RV spot at the usual rate overnight. But for the return trip, I would just need to charge for a few hours in the afternoon, so could I get a discounted partial rate. The person offered, "How about $12 for a 4 hour time?" That sounded pretty good, so I did it. I then created the entry for the place on Plugshare and listed the details of what they had offered for future use. If you look at the listing now, there have been several people later on that used it since it was nicely established. So that's an example if you need to call a new place to discuss usage. You might mention things like how you will not be needing to use their water hookup or waste dump station, so it saves them a bit of cost and might be worth a discount. For those short few hour stays sometimes they'll take 5 or 10 dollars, since it is truly just extra money for them in not taking up one of their overnight reservations.
For others that have been listed in Plugshare, it varies some. If you look up the two in Riggins Idaho, which is a destination for whitewater rafting, Big Eddy RV park says the owners are big supporters of EVs, so they will offer 14-50 outlets for free. The other one in town, Riverside RV park, says they would like $5 for an hour to hour and a half kind of time.
Make sure to call ahead to check policies or make a reservation. Some places may be full if it's in a busy destination, and I have heard a few stories of places that just have really stubborn or ignorant management that think an electric car is going to blow something up or start a fire if it tries to charge, so they just won't allow it at all.