Thegrunt - Big subject and you have not been very specific. A small diesel like a Mercedes one costs about the same to maintain as a big gas motor. And when you add in the better fuel miles per gal on the diesel it is likely cheeper than a big V8 or V10. But maintenance on a big diesel like a Cummins or CAT 7-13 liter motor compared to a big gas, the diesel costs more to maintain. But a BMW gas motor costs more to maintain than a Chevy gas motor of similar size. Cummins and CAT motors are precision pieces capable of 500,000 - 1 million miles before overhaul. So more precision motors cost more than mass production gas ones to buy and to maintain.
In general a diesel motor is really the best one for a motorhome. Lots of torque which is what you want when you drive an RV. The other big benefits are that diesels generally at at the back of the coach so much quieter and the diesels come with air ride.