560 coolant issue
The coolant level sensor is easy if you have the right pliers. The ones that push clips apart.
My brothers 560 kept fluctuating on the temperature. We thought it was a bubble, but one of the hoses had a pinhole in it. We have a coolant pressurizer, and we pumped up the system and coolant shot out and we found the hose.
If the coolant tank, cap, radiator, and viscous coupling on the fan are new/working properly, then I would look at the thermostat. Since you have been through six gallons of coolant, I dont think that you have air bubbles in the system.
See if all your connections on the coolant tank are secure.
Try the thermostat next.