What do the Starship Enterprise and toilet paper have in common? They both circle Uranus looking for Klingons. Thank you, thank you! You’re great! I’ll be here all week.
Between Swift and Knight, they have 25,000-30,000 trucks on the road. The cost to install and maintain that many APU's would be astronomical. It is obviously not cost effective, or you can bet they would have them on all of their trucks.