Fuel mileage diesel vs gas

I have a Navion 24G on the Sprinter chassis. I have to add a 2.5 gallon jug of DEF roughly every 5k miles. It costs about $12 at Walmart. One nice thing about the extra mpg is the longer distance you can go between fuel stops. My old rig, 1997 F250 with the 7.5l V8 and a Lance camper got about 8 mpg and held about 35 gallons of gas so I was looking for fuel starting at about 200 miles from full. The Navion holds 26 gallons of diesel and gets 15 mpg so I can comfortably go about 300 miles before I start looking ( I don't like to get much below 1/4 tank). And at 14k miles per year I do believe the extra cost of the diesel is worth it, not that I have to justify it to anyone but myself.