It is living full time in an RV. Beyond that the definition gets a little fuzzy. In my thinking there are other criteria to being "Fulltime". First caveat is not having a home to return to, your home is your RV. Otherwise it's an extended vacation with the intent to return "home". Secondly, the RV must be mobile. If it is stationary, i.e. permanently parked in a RV or mobile home park, it's really a tiny house. Fulltime includes the ability, and action, to pick up and move as the mood strikes. Third, to be Fulltime, as I think of it, it should be a choice rather than a necessity. If you're thrown into RV living by financial difficulties, or say your house burns down and you live in your RV while rebuilding, is outside my definition of Fulltime.
So who are these Fulltime RVers? Well, the range is vast even with the caveats above. Most are retired. They have the easiest time moving to Fulltime in not having to hold down a job. Although many retired Fulltimers have temporary jobs for fufillment and additional income. One growing sector are younger workers living Fulltime. Many hold down part time (or full time) jobs but others work remotely. We've met several Fulltimers doing software development where location isn't important, just a good Internet connection. Writing and photography are other possible occupations. There have always been construction workers who move from job to job but they may have a sticks and bricks home.
The style and cost of Fulltime RVing varies drastically. You can spend a great deal as a Fulltimer. There are very fancy RV parks with very fancy prices, pushing $100 or more per day. Add in some nice meals out and entertainment. Expenses can be high. But on the other end, many folks boondock (almost) exclusively and cook simple meals in their RV. Expenses can be measured in hundreds of dollars per month. (Boondocking is camping for free or nearly free, usually with no facilities like electric, water or sewer. Examples include BLM dispersed camping, Forest Service campgrounds, and Walmart parking lots.) Most Fulltimers are between these extremes mixing boondocking and RV parks (not the fancy ones). Staying at a park for a week or a month reduces camping fees significantly. As a general rule, Fulltime RVing will cost a similar amount to what you're paying in a sticks and bricks home.
Fulltiming is not a vacation. A vacation is a week or two in to which one crams as many activities as possible. They are a break from life. When you travel for several months this break neck speed starts to unwind. Most people, faced with month long RV trips, start out travelling long and seeing broad swatches of the country. After a few trips, or a few months, life starts to unwind. You can then slow down and enjoy where you are. Fulltiming is an extension of month long trips with more freedom, you don't have to return home because you're always home.
Fulltime RVing is simpler because you have a lot less "stuff". But it is more complicated because you have to consider where you'll stay, find places to dump tanks, get water & propane, shop for food, etc. When you collect something (buy or find) you must ask yourself where would I put it and is it worth the space.
Fulltime RVing is simply another way to live. A different adventure.