Welcome to the forum!
Our Windsor had the same coolant sensor and Volvo expansion tank with a green cap. The new coolants coat the aluminum sensor and cause the light to come on. Remove the aluminum sensor and clean it with fine steel wool or Scotchbrite. I used to keep a spare sensor and as soon as the light came on I would swap sensors.
We even put one sensor in the lathe and bored a hole into it to increase surface area. It lasted marginally longer.
You may have to drop the tank slightly to get the sensor out of the top. It's pretty easy. Three 9/16" hex bolts and nuts.
Be careful not to overtighten the sensor and crack the plastic tank.
You will find a wealth of information in the Monaco Owners section of this forum.
Hope this helps.