Have you considered a B+ like thor makes on the ford transit platform? Those come with the 3.0 power stroke. My in-laws have one that they enjoy quite a bit. It gets 16+ mpg. I do tease them, though and tell them that when they get tired, I'll put it in my Seneca's storage bay and they can ride with us!!
'14 Jayco Seneca 37fs
20' Look Enclosed Trailer