I think that for the minimal amount of time you want to spend on the internet, you may want to head over to Walmart and talk with the Straight Talk people, back where they sell cell phones. You would need to buy their mobile hotspot and also a (refillable) data plan. They will be about the cheapest around BUT it will not be the fastest speed.
* I should add that, yes, many campgrounds have WiFi available. Sometimes it's pay as you go (Tengo) and sometimes it's free (and mostly slow). Sometimes it's available only at the office or clubhouse. There are also other free places such as public libraries, McDonalds, etc. but that means you will need to take your computer to them. But if you're going to pay bills online or do anything where you want a secure connection, then you may want your own hotspot.
Tim