On a typical road train, I think it’s more like three or four.
And so does just about every other language.
Long words are less frequently used than short words, in natural languages. Longer words or multi-word phrases, as they begin to be used more frequently, become shorter.
Thus television receiver set becomes television becomes TV.
In the science of linguistics, this is known as Zipf’s Law.
Remember that the truck names in the song referred to the trucks themselves–“Cab-over Pete” was a cab-over Peterbilt. The song also mentions “a Jimmy hauling hogs,” which would be a GMC truck.
As for fuel capacity, I seem to recall that 900 liters (about 238 gallons) was a typical full tank back when I did some driving. At least it was on the trucks I drove (I didn’t drive reefers).