You are forgetting something... add a 4K trailer to the 11K you have increased the combined weight by 37%. You only increase the other by 15%. How much is the MPG of the MB engine going to decrease when you add 37% to it's work load. 4K behind most DPs decrease the MPG by half a gallon or less. What is the MB going to get?
To go to 25' instead of 34' for better MPG and smaller tanks is short sighted at best. Most do not put enough miles on their MHs so the difference in fuel cost is what? Using 6K miles/year, diesel at $2.40 and 15 mpg and 10... $960 in the MB and $1440 in the Cummins a fantastic savings of $480/year to worry about making it up the mountain and have to go dump twice a week (instead of once a week) when boondocking. It's more likely you will boondock with the DP which will save you more than the $480 in fuel each year.
I always advise buying more MH than you think you need... ONE TIME. It cost me a fair amount of $$$$ to learn this. Your mileage may vary.
PS: Get the best MPG on the vehicle you put the most miles on and stop worrying about the MPG of a MH.