Assuming the gauges are actually accurate the new engines as quoted run hotter to burn off the bad stuff and the DPF's or DEF systems are there to burn off carbon residue. These systems also cause heat retention however the coolant temperature is the restriction in running diesel engines hotter for better efficiency and power. Adding 6000 lbs will slow the vehicle on a grade slightly but only affects the cooling system in cycling times. Allison transmissions should be run in "performance mode" on long hills to assist cooling by raising shift points which as stated increases coolant flow and maintains higher air movement via the fan through the radiator. If you are still running to hot manually backshift a gear and run the engine on the governor which decreases the load on the engine with max cooling.
