I would put a simple toggle switch on your dash to control the fan manually. I did this in my MH and when I see a hill I turn the fan on and I don't have to wait for the water temp to rise before the fan comes on. You don't say how old the car is but thermostats do go bad and radiators cores get blocked. They sell HD stats with a small bypass hole to allow more water flow. You can check the radiator with an IR gun to see if it is blocked from top to bottom or side to side in a grid pattern depending on where your inlet and outlet are located, cold spots would indicate blockage. I don't know why but they seem to block in the center of the radiator.
