I don't know why you guys are taking the fan off. I remove the screen, carefully prying one tab at a time, and wipe both sides with a damp paper towel. Then use the same towel to wipe the blades and I'm done. About five minutes of "work". Maybe you're waiting too long to do it and the dirt build up it pretty thick.
These fans move a lot of air and should be cleaned often. Don't forget the screens in your trailer windows. Air out equals air in. Each one of them is a filter in it's own right. The cleaner you keep them, the more efficiently you remove heat in your rig. I calculated the Two fans in our trailer can change-out the total air volume about three times per minute on high. That's a lot of air!