If you have the Baja California AAA map or the Baja Almanac (topo map book) the stations are marked and either have G or GD, the latter indicating they carry diesel. The longest gap is from El Rosario to Guerrero Negro - 225 miles (and Villa Jesus Maria 15 miles N. of GN may have diesel). Occasionally stations are out of gas and/or diesel. So everyone recommends you gas up if your down to 1/2 and have a chance to fill up.
Just got back from a month long trip and we did go to Mag Bay, Diesel was easily found all the way from TJ. The station in Mag Bay has diesel also but it's pretty tight gettin in and out if you are towing, plus it's kinda hard to find.