Can a truck be repossessed with a trailer hooked to it?

Only if the repossessor has an order for repossession of the trailer, as well. And even then, only under certain circumstances. If it's a commercial tractor trailer, and there's a load in that trailer, they may not take it, as the order of repossession does not cover the load, and they will face criminal charges if they do such.

In the course of repossessing a vehicle, the repossession agency may not enter or move any vehicle (including a trailer) which is not in their order for repossession. They may detach a trailer from a truck being repossessed, but they can't actually take it.