While driving to a trailhead near Ouray today my Jeep overheated. Oil Temp reached ~300F and I lost a lot of fluid. No MIL's until the oil temp spiked.
I was able to add enough water to get it to the dealer.
Is the fluid in your overflow tank creamy or sludgy?
If not, it could be something as simple as a badly installed or just simple a faulty pressure cap. If the coolant is discolored it could mean that the system over-pressured due to a head gasket leak.
One simple problem may be air trapped in the line. This may never rear it's head until you are in a low air-flow situation (stuck in traffic on a hot day, or putting extra load on the engine by towing or off roading).
With the engine COLD, raise the front end of the vehicle with a jack. Then open the pressure release valve on the top radiator (again, do this with the engine COLD!!!). Turn on the engine and let it come up to temp. You may see a few bubbles come out of the pressure release valve. When the vehicle reaches temperature, turn it off and close the pressure relief valve. Lower the vehicle off the jack.
This process should allow any air trapped in the coolant system to escape.
I hoped he, at least, pressure tested the system.
FYI - We've towed with our JGC Diesel in 90+F temps on some decent grades with never a problem.
As the other poster stated, air in the system IS a possibility but usually occurs after a coolant fluid R&R.
The first sign on a new vehicle is a sudden drop in the coolant overflow tank fluid level.
I think I got an entire bottle of Purple Ice in my overflow tank (while cold, of course,) and it still just rests below the max fill line in the overflow tank. Surprised that it wasn't fuller than that from the factory.