Often times a trailer that is set up on a "permanent site" at a campground and used during the spring/summer/fall season is referred to as "not lived in" even though it may be "slept in and cooked in" every weekend during that time frame. Many people refer to that as "camping" not "living in" the trailer. So, as a matter of "terminology" he may be telling you what he believes is the truth.... That trailer may have been "slept in" 100 or more nights every year adding up to over 2 years of "full timing" and still be referred to as "camped in" but not "lived in"
Depends on how the current owner wants to explain it, so look at the amount of what looks like wear to carpet, countertops, molding at the doorways, etc. If it looks worn more than you think it should, then make your own determination on how "used" it looks. After all, it's your money that you're spending, use it wisely.
As for the tires, if they're over 5 years old, don't count on them to be serviceable, they probably aren't !!!!!
Good luck in your search, there are lots of GOOD RV's out there, don't settle on something your subconscious is ringing alarm bells over !!!!!