With our 35J we have averages 6.4 over about 30,000 miles towing costs about a half MPG. If I could have essentially the same layout ina diesel, with a 10k tow, air ride, larger tanks, bigger batteries I would look at it, but do keep in mind you say you'll only use it maybe 60 days a year. Also consider diesel is $0.70 or so more than gas so a 20% better mileage is not saving anything.
Many of the DPs travel twice a year, North to South and back, maybe only 4,000 miles a year, often less. We run over 7k a year and would do more if we could.
