On some class A's Winnebago has an option called MotorAid that circulates engine coolant through a heat exchanger on the water heater and a small radiator with a fan that provides heat in the rear of the motor home while you are driving.
There is a high - low - off switch on the dash that controls the fan.
I don't know if the option was/is available on class C's or not but if so that might be your problem.
Clay WA5NMR - Ex Snowbird - 1 year, Ex Full timer for 11 years - 2004 Winnebago Sightseer 35N Workhorse chassis. Honda Accord toad.