I was having a problem with my motor running rough on my 2007 Suncruiser 38J. It was most noticeable at part throttle and about 1700 to 2000 rpm. My mileage was down as was power. So I took it into a service center. It turned out that I had two spark plug wires that were burned and two others that were in the process of failing. The service center called Workhorse and got the authorization to have two ram air ducts installed to cool the sides of the motor. These are scoops behind the lower grill with clothes dryer size hoses that run up to the motor and blow cool air on the spark plug area on each side. They replaced all eight plug wires and rerouted the cables also. This has solved my problem and my motorhome is back to its usual power and mileage.
Apparently the rubber air dam and the doghouse trap too much heat which attacks the spark plug cables. By they way, this was all done at no cost to me. I am posting this in case there is someone else out there with a similar problem.