Went from a 27' 2011 ford v10 gasser to a 35' 6.7l DP. My gasser could pull the grades better than my new DP. It's more than just the engine, you also got to factor weight. That being said I love driving my DP as it is quiet and handles way better than my gasser. Weight difference on the two was 10,000lbs. My gasser was lucky to get 7mpg while my DP gets just over 9mpg. That difference overcomes the price of fuel.
So when you compare DP to a gasser you need to be more specific as the the actual models vs just looking at the engine. In general almost all DP will be more comfortable to drive and if you do a lot of miles that should be a huge factor. Otherwise floor plan and the condition of the RV should be high on your list.
