I'd probably call a "tractor-trailer" an articulated lorry or "artic". The front bit would certainly never be called a "tractor". more likely a "cab", I think.
Not quite true, shona - among professional truckers (a USn name which has become common over here in the last 20 years) the term "tractor-unit" is fairly common, and this is sometimes abbrev'ed to "tractor." It's also called the "prime mover." But "artic" is the usual - indeed, almost the only - term for the whole contraption.

a wagon is always pulled by a horse or a team of horses. or a railway locomotive. (But increasingly the term is being used, in the USn sense, for a lorry )

a minivan is ...errr.. a small van. the name originally springing from the commercial version of the Morris Mini-Minor, that wondeful invention of Alec Issigonis. But now, as shona rightly says, applied to almost anything with a capacity of ½ ton or less.

Brits wouldn't have a clue what a panel truck was. now, I have to disagree with that one, except that we say panel van, rather than truck (a truck is more likely to mean a hand cart with four wheels over here, BTW) Again, the term has only come into vogue in the last ten - fifteen years, but they are advertised as such in the press. I suppose that, like the use of "tractor" for the front bit of an artic, "panel van" is better known among commercial drivers and operators than the gen.pub., but the term is definitely in general use here.

SUV - doesn't that stand for "Sports Utility Vehicle"? (to me, a contradiction in terms, but I travelled for many comfortable miles with Jackie in one this summer, so what's in a name?.)