I have a similar experience. Talked with Cummins NW about it. Their explaination was: WRV had two different versions of the 8.3 C engine, the 275hp version annd the 325hp version. My coach has the 325hp version. The air box on the coach is the one designed for the 275hp version. This can lead to the air restriction indicator providing an inaccurate reading, I was told the engine is getting the proper amount of air. I get 8.9 mpg with tow at 60mph. This is right with what others report for fuel mileage with the 8.3 C engine, so don't thing I was being led astray. I do change the air filter about every 10,000 miles though, soomer if I see a drop in fuel mileage. Don't know what WRV did with 2000 model year coaches, but something like this could be.
'99 Alpine Coach