Here in San Diego diesel has been the same or lower than gas (regular)for more than a year.
My 2003 Pleasure-way Excel TD, 8 cyclinder, got 11-12 mpg. The tank was 26 (?) gal. (can't recall right now.)
My 2006 Pleasure-way TS Sprinter, inline 5, has been getting 22.5 mpg consistently. It has a 26 gal tank.
So I get twice the miles per gallon with the Sprinter than I got with the gasser. The biggest benefit for me is that I get 600 miles per tank which allows me to take heretofore unexplored routes without worrying about how far I am from the next gas station.