A few more details would be helpful to get good responses specific to your situation.
For example,
What is the year and engine model of your Newell?
Exhaust emissions rules have changed through the years, and different years and different models of engines have different "normal" smoke characteristics.
When do you see black exhaust? Mostly just initial acceleration from idle? During full throttle acceleration on the highway? All of the time?
Typically, black exhuast indicates that the engine is receiving more fuel than it can thoroughly burn. Excess fuel produces black smoke indicating less than complete combustion. Some engines normally produce some smoke on acceleration and during "full throttle" pulls on the highway. In other engine models and years, black smoke is indicative of an incorrect adjustment or wear.
Better advice would be possible with some additional information. That having been said, you most certainly would benefit from a thorough inspection of your engine by a qualified, factory-trained diesel mechanic. We suggest considering the service department of a conveniently located distributor for the engine brand installed in your coach.
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Hopefully this helps.
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