This is what I base what I know on. 03 E350 with 6.0 turbo PS loaded with about a ton of material in the back, and then the trailer with the lift which was easily another ton- that's only 4k lbs. I saw 20 mpg without the trailer, and 17 with the trailer- all this as fast as I could go on the highway. With the 02 5.4, we never saw over 10 mpg (no trailer on that trip, about the same material in the back), and were always stopping for fuel. I also spent a short time in a Duramax 3500 van with a non-turbo diesel. That one never got over 15- hence I believe that diesels should never be without turbos.
Now the hybrid diesel vs hybrid gas is a completely different thing. The diesel engine is better suited for turning generators than gas engines.