If this interests you, please see https://touringfreedom.com/blog/vending-at-fairs-and-festivals-while-traveling/ for an excellent article on doing this and what is required. Now that it has become so easy to accept credit cards anywhere via smartphone and tablet dongles for Paypal, Square Up, etc., there is no reason you can’t make good money selling handmade crafts and purchased products. I would add that once you have a business setup in your place of residence with a tax resale certificate in hand, it is possible to buy goods wholesale or purchase from the many liquidators spread out all over the country. The key is in spending the time to do your research to find out what people are buying and what people are buying at a particular fair or festival you may be interested in. I would attend a few festivals/fairs before getting started to get an idea of what is selling and what is involved. Perhaps later in the afternoon near closing time when the crowds have thinned you might speak with some sellers to get their take on this lifestyle.
Don’t forget, the purpose of this lifestyle is usually to travel and see new things. Don’t get so caught up in traveling to the next fair that you forget to schedule some time off to see the local area. Remember why you wanted to become a full-time RVer in the first place.