I have a 1994 S10 6cyl. and it gets an avg. of 23mpg, but I don't drive it hard and I'm in the south. No mountains.
The average full sized diesel pickup truck gets about 26 miles to the gallon which is about 10 miles to the gallon more than a gas truck. This is due to the fact that the diesel engine is far more efficient then the gas.
13 mpg
A 1989 Chevy Silverado Pickup Truck will get between 14 and 16 miles per gallon. This does vary slightly from person to person, depending on how well the truck is taken care of etc.
In a pickup truck you might get 10 to 14 miles per gallon if you drive conservatively with a 7.4 motor. The same motor in an RV might be lucky to get 7 to 10 miles per gallon.
The gas mileage depends on the model of pickup in question. A few get as high as 26 highway miles per gallon, while others drop as low as 16 highway miles per gallon. This mileage is worse in city travel.
20 miles to the gallon 20 miles to the gallon
15 miles to the gallon 15 miles to the gallon
it can get 8 miles to the gallon
18.5 miles a gallon
16.5 miles per gallon
52.27 miles per gallon
About 21.4 miles per gallon. (300/14 = 21.4)