Can a repo man cut a lock holding a trailer on to a truck up for repo?

Each state has different laws regarding repossession so it would depend on your state.

It is true that every state has different laws regarding repossession, but this might fall under the breaking and entering statutes for your state. Very difficult to say without knowing what state and having some legal knowledge. However cutting a lock would be considered very bad form almost anywhere. Further if the repo-man simply takes the broken lock and repossesses the truck it will just be his Word against the debtor, and it will probably go no where. Unless there is a security camera.