I have a 2009 Chev G3500 6.0 gasoline van. I have about 45k miles on it so far. Empty holding it to 60-62mph I get upwards of 19mpg. With flooring, etrack, insulation, paneling, clothes, fridge, food, bedding, books, straps, full fuel etc. etc. I can legally carry about 2900 pounds. I told my carrier I can carry 2500. With a ~2100 pound load and having to run 70mph to make a super critical delivery time I get about 16mpg. My overall is about 17.2mpg but I use 16mpg in my spreadsheet for calculating purposes.
I run full synthetic oil and get about 9k miles per maintenance interval cycle. I replace the oil filter and add 1qt new at the first interval and do a complete oil/filter change at the second cycle so I'm getting 18-19k from 7qts oil and 2 filters. I could get better mpg with a Sprinter but would pay more for fuel so the difference overall isn't that great. The advantage of the Sprinter would be standing up straight, taking taller freight and taking 3 floor spots at times. I'd have to do a lot of figuring to determine if the higher entry fee and higher expenses of a Sprinter make sense for me. They absolutely do for many and probably don't for a few.
Good luck with your research.