Forgot to add: the truck is running great other than fuel mileage.
A leaky exhaust manifold can definitely cause a drop in mpg. They can leak both or either direction, and the one that causes mpg problems is during the phase where outside air gets sucked into the exhaust stream. The computer assumes any air in the exhaust is from the cylinder, meaning there’s not enough gas injected to burn all the air. So it will inject more gas if it senses extra unexplained O2 in the exhaust stream.
Cold weather and maybe winter mix gas,I loose 4 mpg in winter for an 03 160k miles, and still running strong.
I suggest replacing the oxygen sensors as they typically start to degrade around 80-100k. I have replaced several although there were no codes and the cars drove better and mileage went up dramatically. It seems one has to cut the wiring to an O2 sensor to trip a code from what I have seen. Also, the exhaust will smell very rich. Definitely check your thermostat and temp sensors too.
One thing you will notice with a sticky thermostat (stuck open) is that the temp usually swings quite a bit as you drive. It will get hotter when sitting at idle such as at a stoplight but really drop at easy cruising speed (45-55 mph).