It probably depends on the reason for the sign. If the idea is to keep people from camping on the side of the road, you're an RV if you camp on the side of the road in your RV, you're a pickup truck if you park for a few hours while you go for a walk. If the idea is to keep scrappy looking campers out of a neighborhood, you're an RV no matter what you're doing (some neighborhoods just hate RV's). If the idea is to keep people from camping in the Walmart parking lot, you're an RV if you camp overnight, but you're a pickup truck if you just go in to shop and the camper happens to be loaded.
The guy who writes your ticket really could care less if you have a truck camper or a class C, and can't tell the difference. Big box on the back of a van/truck, windows, door, maybe a bike strapped to the ladder, it's an RV.
Brian