Jim, congratulations on your new motorhome. From what you described that is absolutely normal for a diesel. If I don't wash the body around around the exhaust pipe on a trip in my MH it will get sooty black. The sooty black also creeps down the outside of the actual exhaust pipe. The time to watch your exhaust through the rear view mirror is under heavy acceleration or when climbing a grade. It is normal to see a little puff of black smoke when you floor it and when it shifts gears but that should be all. Your MPG appears to be normal. If the exhaust pipe is crome then get some chrome polish and clean it up and protect it.
"Gunner" USN Retired, Airdale
2004 Monaco Signature 44' Conquest. Detroit 60