Full sized truck dealers are like any used car dealer

The biggest difference is how much mark up there is in a used tractor ( Semi). As far as new goes heavy truck manufacturers have concessions (discounts that vary according to the size of the fleet. Prime , Arlo Lott etc… get huge concessions. Maybe upto 20k per unit or more depending on the size of order and how many units the have in operation in total .

Prime may order a 120k tractor and only pay 85k for it . Then they hire / train a driver on the condition he/she stays on with the company for a period of time . Then they lease that tractor to the driver at a cap cost of 120k + % ! The driver is essentially an indentured servant and prime is making bank on the lease + getting cheap labor ! I used random numbers as an example but the actual numbers are pretty similar in range .
I managed Volvo Truck sales department and sold for a Freightliner dealership back around 2000.