Ya, $4,000 for a fan hub bearing (parts and labor) is CRAZY. I guess if the coach maker gave you no access to the top of the engine, then the radiator and CAC would have to come out-- that could run $4,000 if they have to dismantle the back of the coach to get to it. If that is the case, shame on the coach builder.
Now, if it was let go so long that the fan went through the Charge Air Cooler, THAT is a very different matter.
As with most bearing failures it would be very surprising for a bearing to go from fine to failed without a lot of noise and increasing play. Said another way, you should be able to hear it and/or feel play before it just "lets go".