Fuel would be around 7.0 mpg average(round trip) if the 550 is running well...plus wear and tear on tires, brakes, etc. I know some bonehead will splash back that he gets 21 MPG...hell you can't get that in a 150 with NO trailer. I have driven 350's, 450's and 550s' for years , so keep it sane. No "Hog Tales".
The coal will be fine, but 'baby steps' may be safer until you are comfortable with the quality.