I have always disliked slippery slope type arguments in regards to our laws/rights in America. We have this beautiful system of govt where laws and regulations can be repealed. Nothing is set in concrete. If the regulations get too severe, then the legidlasture can repeal them or the courts can decide they are unconstitutional.
I think we go overboard, in both directions, all the time with issues like this. One side over uses their power until they get beat back by the other side. Then that side goes too far. But our system is set up to keep things in check.
As for this, trucks and their cargo are already checked in many states. This is nothing new. of course commercial carriers dont enjoy the same rights as private passenger cars. You think when they are hauling fluids or pesticides or anything dangerous that they are not regularly checked? Of course they are.