We got hit up for a new set of 5'er tires in OKC some years back while fueling up at a full service truck stop. I was approached at the fuel island by the truck stop's "Tire Guy" in exactly the same manner the OP was hit up.
The "Tire Guy" told me that as we made the turn to line up with the fuel island lane, he noticed a bulge on the inside curb side rear tire on our 5'er. Upon my inspection, I felt around the entire circumference of the tire and the inside/outside sidewall feeling no bulge and seeing no obvious bulge. "Tire Guy" was adamant though that he'd seen a bulge on the inside of the tire and that I should go talk to their tire manager for a 2nd opinion. "Tire Guy" did get me to thinking that maybe we in fact did have a tire issue. When I pulled away from the pump island, instead of driving over to the tire garage entrance at the opposite end of the complex I parked in the overnight truck lot less than 100' away. I carry an extra spare 5'er tire in the bed of our truck when we're traveling over the road and I have an EZ roll up jack. It took me less than 10 minutes to change the tire in question on the 5'er with the spare I had in the truck bed.
Just as I was tossing the supposed questionable 5'er tire that I had just pulled off our 5'er into the back of our truck, "Tire Guy's" boss came walking over to ask me what size tire I needed to buy to replace the bad tire. He told me that if one tire's gone bad, "Better to replace them all." I told him I had already changed the bad tire with a spare. I then asked Mr. "Tire Manager" to please inspect and spin balance the tire in question. Verbatim his response was, "Only if you're buying new tires!"
We left. We later stopped at a Michelin dealer who inspected the tire in question, gave the tire a clean bill of health, and reinstalled it on our 5'er at no charge.
This scam has been around for a long time. Just like selling a customer their own gas cap, or an empty quart of oil, or a customers own windshield wipers, a customers own headlight bulb, repairing a flat that isn't flat, etc...etc...etc... One just has to be aware that when someone approaches about doing you a favor that you didn't ask for, the red flag should go up.
Happy camping!!! See y'all down the road!!!:)