You should be able to get a copy of the N.V.I.S., New Vehicle Information Sheet from the dealer. This will have the VIN
Use the VIN
to buy insurance that goes active the day you are scheduled to pick up the trailer. I used Good Sam for insurance but there are other RV insurance firms out there.
The dealer will provide a bill of sale but I suggest you also get them to sign a bill of sale form that you can obtain from your state Department of Motor Vehicles or equivalent office. Use the currency conversion rate on the day of purchase or day of money transfer to show the US$ purchase price on your state's bill of sale.
When you stop at the border, show them both bill of sales and they will inspect the traielr and give you a Customs Form 7501, Entry Summary. They will use your state's bill of sale amount for the purchase price. There is no duty on trailers.
Register the trailer in your home state within the prescribed time limit and you should have no problems.
I suggest you consider using Custom House http://www.customhouse.com/
for the currency conversion. You can wire transfer money directly from your bank to their Bank of America account and then pick up a Canadian bank check at one of their offices. Their exchange rate looks good and they give better rates for large amounts. By wiring the money ahead of time you won't have to guess what the final cost will be before you travel and you won't have to carry a large amount of money or checks. Custom House should let you transfer the money 3 days ahead of your scheduled pick up date.
You will be charged Canadian tax (GST) when you buy the trailer. When you stop at the Border for US Customs you will also need to go to the adjacent Canadian Csutoms office and get an export stamp on the dealers bill of sale. After you register your trailer in the States then mail the original dealer's bill of sale with a GST refund form (available at Customs) to get the GST back. If the GST is under $500 Canadian then you can an immediate refund if there is a Duty Free Shop at the border. I imagine your GST will be above $500 so you will have to mail back the original bill of sale. If you lose the bill of sale, you lose the 7% GST refund! You may need the original bill of sale for registration
in your state.