National Interstate had a definition: More than 150 nights/year in the RV.
In the absence of a definition form the insurer, I would use a common sense approach. If I had another, second, home (owned or leased), I would say that obviously I am not "full time" in either one and thus not a "fulltimer". If I did not, I would have to say the RV is my "full time" home. Staying in a motel or with a friend is considered a "visit" rather than a residence.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL