Sorry to hear the news, flatcat. The hubcaps can be glued in place with two-pack epoxy resin. That's the system I used with my 4082 THW mega-set.
Did you buy off eBay? If so, the seller may offer a returns service for cases like this. Also, if you paid with PayPal, then you are protected against damaged goods. There are procedures to follow, e.g. raise the problem with the seller first, however. Let me know if you think I might be able to help in anyway.
The bodywork of the towing vehicle is injection-moulded in blue plastic so the white may well be a scuff off the boat trailer. (On my 4821 fire engine there was a red scuff on the white roof.) For a darker colour to scuff with a lighter one is unusual, but if it is a scuff it should polish off using T-Cut, a paint restorer for cars. If it isn't a scuff, then it sounds as if it may be a stress mark and that won't polish out. Don't try any of these until you have tried to solve the situation.
If the worst comes to the worst, you could always spray paint the area concerned, either by disassembling the model or masking off. I have a can of what should be the right colour which I used for this custom:
http://www.playmofriends.com/forum/index.php?topic=2664.0