I have an 06 Winnebago Adventurer 8.1 on workhorse chassis bought it new, and when I first got it I went through about 3 sets of plug wires, then I found out it was due to not enough air flowing to the back side if the engine. It was while it was still under warranty I bought the equipment to duct the air back to the last 2 plug wires on each side of the engine. They installed the duct work for free.
It was told to me that when the coach was placed on the Workhorse chassis they did not allow enough space to let the air get all the way back to the back plug wires. Since then I have not had any problem with the wires.