Our rig did that too after we purchased it. I think it may have sat for a year or 2 without use. It would literally coat our toad with a black/sooty/oily deposit. After changing the fuel and air filters, and a couple thousand miles, we no longer had the problem. I also started using Lucas Injector cleaner on every other tank of fuel. Our rig has the 350 Cummins.
Maybe just needs to stretch it's legs a little.
2000 Fleetwood American Eagle DP
2010 Cadillac SRX, 4 bicycles
Fulltiming since Oct. 2, 2010. Kerry & Bess Ann