Jose,
You don't state the year or chassis your Class "C" is but I have the Ford 460 and when I first got it I could tow my trailered dune buggy through the mountains with no overheating but lately it has been heating up climbing mountains without anything in tow. Did some reading and decided to check the fan clutch. The fan clutch only fully engages when the vehicle is actually heating up beyond the normal operating range. Before then it partially engages but is pretty much coasting. As the temperature rises the spring on the clutch restricts and this opens a valve in the clutch allowing the viscous fluid to move to an inner channel which allows the fan to more closely match the engine speed. This accounts for why it roars when engaged. It is pushing a lot more air. I put a piece of cardboard with a hole in the center in front of the radiator to restrict airflow except for where it passes over the clutch. This is because you have to have the heated air flow for the clutch spring to compress. Once the engine started overheating the clutch never engaged, so I am reasonably sure that is the problem. If it had engaged the amount of noise from the fan would have risen sufficiently to easily hear the difference. The clutches are not that expensive, $35 or so, but the labor to change it can be extensive, including possibly needing to have to remove the radiator to gain clearance. So the test is a worthwhile step to verify if the clutch is operating normally or not before replacing it.
