Full blast is default on the fan which means that the solenoid on your fan motor is probably either bad or not getting a signal from the computer. Some are regulated by the ECM and others by a thermostat mounted on the radiator. Firstly, I'd check the wires going to the fan motor and see if you can detect a voltage signal. If no signal I'd try to find out why. I had the same problem and finally found a chafed wire in the harness.
As a side note, after I fixed mine, the fuel mileage went from 6 mpg to 7.5 mpg.
2005 Newmar Dutch Star 4024
Pulling my 2012 Malibu w/Blue OX & Patriot brake unit