I am afraid that there is no accurate answer to your question. There are too many factors that can affect the lifespan of Motorhome engines. So I am going to give you some general answers to your question.
Generally a well-maintained Diesel Engine has a greater lifespan than a well-maintained Gas Engine due to how it is designed. There are people that will say that a Diesel Engine with 90,000 miles is just getting broken in on the other hand a gasoline engine with 90,000 miles might be getting close to needing some major maintenance to get it to continue to operate efficiently.
Well-maintained engines regardless if they are diesel or gas will last longer than a poorly maintained engine.
Based on your question I am going to assume that you are looking at buying a used Motorhome with 90,000 miles on it. If that is the case there are many more important factors to consider before making a purchasing decision above and beyond how many miles are on it.
I urge you to visit the Used RV Inspection Guidelines Section of our website to get information on all of the items you need to look at on a used RV, so that you can make an educated buying decision.
I hope that this information has been helpful to you.
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