Well I wouldn't run my truck 1 gallon low. For several reasons: one being that this engine uses coolant to reduce temperate of the charged air from turbo and two it is also the primary means of cooling the oil as well. If you reduce the volume of water in your system you increase the operating temperature. So since the coolant isn't just operating in the engine block itself, it also cools the incoming charged air, and oil as well it is very critical to maintain the coolant at the proper levels.
Also the oil is being used to cool the turbo, so if you have less coolant in the system that means when the oil reaches the oil cooler it will increase the overall temperature of system.
Things I would do:
One change the oil, new oil like Rottella T is a good choice. It will provide better protection against heat and transfer heat better in the oil Cooler.
Two, I would make sure the Coolant is both properly mixed for the ambient air temps I would be likely to operate in and secondly at the correct levels.
Three, check the thermostat yours might be shut off a bit.
Four, make sure all of the truck's panels are in the correct place the air is designed to flow in a specific manner if it doesn't the truck's temp can rise.