How do you get into the semi truck repo business?

First, get some substantial time driving them. When you repo one of these, you have to be quick - if it's attached to a trailer, you typically have to drop that trailer, and it could be any type of trailer... dry box, RGN, whatever - and again, you need to be quick. None of this OTR, saunter out of the truck singing, "I've got tiiiiiiiiime on my side" nonsense. Then, you get enough experience that a towing and recovery company will hire you. Get a little time doing that, let them assess if you're ready to do repo work. If they decide you are, then you get into it.