Keith:
We had overheating problems on our 2001 Marquis Amethyst C12 as well. I had installed the metal reservoir and serviced it appropriately. When we left on our 2014 road trip, we had a small leak in our heater core for the driver and pass heat and did not realize this. As our travels continued, I started to see unusual temps but would settle to normal once we were rolling on the interstate. Eventually, the coolant got low enough in the radiator and our temp issues were compounded because the radiator fan probe was not sensing any fluid temp and therefore the fan would not operate appropriately. When we were experiencing the temp fluctuations, I had made sure the reservoir was full but did not continue to check everyday because it wasn't taking that much fluid. After the heater core was bypassed it took several days to get the coolant filled because the engine needs to heat up and cool down. On my last run to the cat dealer, the coach maintained an acceptable temp, but due to the temps I reached (225 deg.), I had them change the thermostat and engine oil and all was well. It took several days to fully top off the coolant even after the Cat tech was complete.
I thought I was servicing the reservoir correctly but I should have been on top of it every cool down.
As Gerald said, make sure the system is full.
Good Luck Larry D.