Yes, there is an excise tax to be paid. If it is not paid by the US seller, the buyer will have to pay the tax.
Without paying the tax, I don't think you can get tags for your truck or trailer.
It is supposed to be paid by the retailer.
Same requirement if the seller is a non US seller?
So the 12% FET would be paid at the time of registration of the trailer?
The tax is due at the time of the first retail sale in the US.
This tax was due to expire on 9/30/11, but I think it was extended by Congress.
Publication 510, Pages 37-40 deal with this type of tax.
Since you did not file an exemption (nor would you be qualified for one), you would need to pay this tax, assuming you are the end-user and you bought it directly from the truck manufacturer.
This is a retail tax and is reported on a Form 720