Was this in hot weather or cooler? Have You pressure tested the cooling system? & cap? & did you remove the thermostat when you reverse flushed? Only takes a pin hole to lose pressure & overheat & towing slows the airflow especially , did you bleed all the coolant bleeders? & check for any tiny leaks near the 2 'O 'rings that are in the tensioner area , sneaky culprits sometimes, look for any tell tale discoloration or corrosion , & weakened heater plastic component & softened hose, under clamp, it's a pain finding the issue a lot of the time, I had a VT Calais with an L67 engine & It did my head in for awhile too, good luck