I have a 2004 Tahoe with a 5.3L V8, currently at 180,000 miles on it.  This issue originated when it took almost 18 miles of freeway driving before the thermostat would open and get hot inside. I changed out the radiator cap since it was the original, and I then noticed that the coolant level was low in the reservoir and added approximately a quart of coolant/water mix.   I drove it back and forth to work for a week and it still took approximately 18 miles of freeway driving to get hot air blowing, and the coolant level is down again.  I don't see any leaks under the truck, even checked the lines for the rear heat, and under the dash the carpet is dry.  The oil was at the correct level and wasn't discolored.

Would it be a safe bet thinking it is a head gasket issue?

Thanks

 Mike