Tire pressure sensors

I purchased a new 2013 GMC pickup last year. I thought the salesman told me that tire pressure sensors were in all the tires. Well, guess what? He was wrong. We were working our way home from a 4 month trip to Alaska and we were camped in Medora North Dakota and were in town getting some money from the ATM at the bank. This older gentleman came over to me and asked me if I knew I had a flat tire on the rear passenger side. I replied I hope not because my built in tire pressure monitor did not alert me. Well after we got the flat to a tire shop 45 miles away I had the shop check to see if the sensor was bad. His response was "You do not have tire pressure sensor"......Man was I surprised because I had been putting my faith in being alerted should I have a problem. I have the tire pressure monitors on my 5th wheel.
So I was driving along fat-dumb-and happy.....