There are no standards for this, it is a marketing term that each manufacturer interprets as needed to sell into a particular market.
If you have need for four-seasons use, then you need to know just what conditions that term means for you, and look for the specific construction methods and features that support your use.
What I would look for would be adequate insulation and cooling capacity for hot desert and subtropical humid summer conditions, and for sub-zero winter conditions with all plumbing and water storage in an enclosed, insulated, heated space. Stretching fabric across a space to enclose it, without heating, doesn't work for me, but started to be the standard for "fully enclosed basement" around 2004. Heated waste tanks don't make it, I have those and understand their limits.
On the other hand, I have not much interest in a RV designed for weather extremes at both ends of the climate spectrum, because I'm going to be moving around to climates that are comfortable to me when I go outside. More often I'll be trying to deal with too much heat, rather than too little.
