I've been doing a lot of research on this issue for when we snowbird this coming winter. It really boils down to how you will be using the internet. If you will be just using email, daily web surfing, reading the news or coming here and reading/replying to a few posts, using your phone as your hotspot with an "unlimited" plan would be fine. All of the major carriers cap your phone use as a hotspot at around 10GB per month. Some will allow more use but it's more expensive than a dedicated hotspot plan. If you plan on doing any kind of streaming of netflix or Youtube or Hulu or facetime, skype etc., you're much better off getting a hotspot. Even then, you get capped at 50GB or 100GB per month with most carriers. There are businesses like OTR Mobile and others that rent tower space and offer attractive plans with a lot of data allowance but you get literally no support when something goes wrong. Besides those things, a dedicated hotspot allows for several devices to be connected at the same time and you get longer battery life than a phone provides.
Just to give you a better idea of things...At my home, it's just my wife and me. We each have a desktop computer, we each have a laptop and we both have smart phones that are connected to wifi while we're here. We also have 2 Roku sticks connected to 2 televisions. We subscribe to Youtube TV for television. I spend about 4 hours a day online, reading, coming to forums, watching youtube videos and occasionally I'll have a ball game on in the background while I'm doing work. One of the televisions will be on about 5 hours a day. We use about 500GB of data per month. Yes, that's right 500GB! So it wouldn't be unreasonable to assume we will use 50GB-100GB per month while in the RV.