Floor plan is still the most important thing to consider when buying an RV. You spend more time relaxing in the coach than you will driving the coach. The average mileage put on an RV is still around 4000-5000 miles per YEAR so for most owners, longevity is a non-issue as they will never wear out the engine in any RV.
Maintenance and repairs should also be considered. If you do it all yourself, the DP will still cost more for the parts.
If you break down on the road, the Ford or GM gasoline engines can be repaired by any garage and most parts are available at any auto parts stores. The DP will have to be taken to a diesel shop, and parts will be more difficult to obtain while on the road.
Neither engines are going to break down very often but if you are looking at used coaches, you have to consider repairs.