Jim,
The tires are all 4 years old and seem to be in excellent shape. I had checked all 6 tires a week or so prior to our trip with a gauge and they were all on or about 95 pounds of pressure. They had dropped in pressure due to a considerable drop in temperature. My intention was to top off the pressure on my way out of town. So I don't know when the tire went flat, but I only drove about 2 miles total on it that way at less then 30 MPH.
I shouldn't have thought it necessary to add this, but I now have a framing hammer to "kick the tires" with at rest stops for bathroom breaks. I will certainly be checking air pressures with a gauge before every leg of a trip. I had never considered getting a TPMS but I am now going to research that option.
Thanks again...