Back when I was in college and lived in a "trailer park" you know the kind for what are now called mobile homes, there were several families that came in and stayed for about 6-8 months in fifth wheels or TT's. they all worked for a single company doing electrical work in major commercial construction projects. I guess that was my first exposure to full-time RV'ers, although they only moved once or twice a year.
I think this is much more common now (30+ years later). Several RV parks I have looked at for a place to stay when traveling have advertised "closest RV Park to ????" and naming a nearby plant that requires periodic shutdown/maintenance or doing major expansions.
