This is apparently an all too common problem on the V10. As someone noted earlier, it's been discussed at length on the Jayco board. I just installed the idler pulley (kept the same belt as it looked good and only 14K miles on it) before a trip last week. The noise started up around 9 or 10K on the engine, and sounded like the old low octane valve tap.
Numerous folks have performed this mod, and all so far have stated it works well. I just did about an 800 mile R/T and there was NO noise.
Interestingly enough, there was enough play in the belt that I didn't have to remove anything to install. I just used a stepladder to get my hands down there, and could hold the belt aside enough while I ran the bolt in finger tight. Was then able to use a small drive to tighten. If you wanted to use a torque wrench you would need to disassemble a bit, but as someone pointed out the travel of the belt is in the same direction so it likely won't be a concern.
I ordered the pulley and bolt from Lakeland Ford. While I initially ordered the wrong part, they advised me of the correct one based on my VIN:
This message from Lakeland Ford Online Parts is intended for XXX in regards to order #123456.
Your order has been updated as follows:
We appreciate your purchase with us!
Unfortunately, the part ordered does not correspond to the provided VIN number.
The part ordered: F65Z8678AAA
The VIN correct part: 1L2Z8678AD
The attached pic shows the location of the idler pulley. The location of the engine is threaded, so in all likelihood it was deleted as a cost savings measure by Ford.