I had a really musty exhaust smell in the back of my coach for several trips and thot it was the fact that I always travel with the bathroom vent open,it has a cover for weather,and I was sucking the fumes in that way. I closed it and still had the smell.It was the same smell as I had always noticed in my school bus. Well,I was under the coach draining out some tranny fluid,long story,and noticed that I had an axle seal leak on the left rear axle. Got it fixed and now the smell is gone.That may not be your problem,but you might check underneath for any kind of fluid leaks.
'01 Tradewinds 7390 LTC
2011 Jeep Liberty