You will be limited to campground mostly near cities over 5-8,000 in size. While a lot of the country has cell phone coverage, a lot of that coverage isn't capable of fast internet speeds.
Doesn't really matter if you have Verizon or ATT, you are going to find no cell coverage in places that the coverage map says you should have coverage.
If your income depends on a cell link, look into getting a cell phone signal booster.
As far as data used, I suggest you download and install an app to measure your usage now. Even better, setup your computer now so that it uses the Verizon data plan and see how well it meets your needs
I do not count on WiFi from "free" sources like campgrounds, Starbucks, McDonalds. We have both Verizon and ATT hotspots. Many times we need to drive into a nearby town with our hot spots and computers to check e-mail, banking, etc. we always carry our hot spots.
