No.
250 horse is not average, its totally low.
500 horse is
Back in the 80's I worked on a special project with Peterbilt. Spent over a year half time in one of their factories, which was in Newark, CA at the time.
Big Rigs are very customizable, they can be bought in many different configurations, the same displacement engine can be ordered in a wide variety of hp ratings. Owner-operators ordered trucks with 400-500 hp at the time, fleets bought 175-250 hp versions of the same displacement engines. The only real difference was the size of the fuel injectors.
If you look at all of the tractor-trailer rigs in the US, the average hp is much closer to 250 than 500. Gross weights of the vehicles have not increased. Engines have improved, but hp requirements have not changed. Pete and Kenworth (same parent company) have a higher percentage of owner-perators who buy their trucks. But even in their sales the average horsepower was much lower than most people realize.
The reason is that lower hp ratings make the engines last much longer. Fleets have lots more vehicles than owner operators and longevity is important to them.