You will get stunning gas mileage!
Put a five gallon can of gas in the bed of the truck and it will last 500k!

Now, diesel fuel mileage may be a little different.
My 3/4 ton ext cab short be 4wd regularly hvers at 18 or so around town and I have hit 22-23 on the highway with light loads and keeping the speed down to 60-65.
But 20 on the highway at 70+ is pretty common for me.
Pulling the camper, again depending on speed, I have done as well as 15-16 mpg of fuel.
The bottom line: PSD's have great fuel economy potential; your right foot is probably the greatest variable in the equation.