I have had three diesels now. First was an '04 Duramax. With a cheap programmer and CAI I saw around 24mpg on summer fuel (once on an all expressway trip figured out at the pump not with the computer thingy).
My '93 Cummins saw around 18-21 mpg on summer fuel sometimes higher if it was straight up highway travel. It was lifted with 35s, I tuned the pump myself built a $40 CAI and nice exhaust. It was awesome sold it at 290xxx running awesome.
My current pig is a 2000 Dodge Cummins 3500 dually. With a Air dog fuel pump cai exhaust and edge juice with attitude. I have only had it since September and love it but rarely see 18mpg hopefully summer fuel with new injectors will bump me into 20mpg. I rarely pull a trailer and not one big enough to warrant owning a diesel, but I love it. The durability and longevity of it is worth it to "me". I have never ever had an issue trying to find fuel. But I love diesels.