Diesels cost more, can typically pull/carry more, cost more to maintain, have better brakes, better ride. If you travel a great deal then MPG becomes more of a concern. Want to tow a large trailer or SUV in the mountains then towing capacity is a bigger concern.
Me I have a smaller (31 ft) class A gasser, do not tow, and am perfectly happy. I don't see a need for a diesel in my future.