Your cooling system is working as it should if / when you see variations. They all work the same .... as your thermostat opens ... the engine temp will cool down. As it closes ... the temps will climb a bit ... this occurs to allow the coolant to cool down by being held in the radiator. As the temps come up (engine temp) the thermostat will reopen allowing the cooler coolant into the engine. When temps reach (overall) the temp where the fan(s) kick on .... the temps will come down to the point where the fan(s) will shut down.
The point is ... the variance in temps is an assurance you cooling system is working properly .....
additionally, engine RMPs will cause some change in temps as well, how much coolant is being pushed through the system by the water pump .... thus the reason cooler temps when down shifting on hills .....
Its when you don't see this happening should you be looking into a possible problem with the system ...........
This would apply to any cooling system.... gas, diesel, coach, car, truck, etc.
Hal & Jackie .... 03 TS Select 45DS01
02 Haulmark 20' Edge .... 02 Corvette Rdstr