Not a big problem. You just need a couple of customs forms. Since the trailers aren't that old the info you need should all be there. Bill of sale, registration, import form HS7, and customs will issue you an entry summary form. Then you just register it in your state. Just make sure you have the conversion rate on money paid so that you aren't paying taxes based on a Canadian dollar number. (State charges the taxes.)
It took maybe half an hour at the border crossing at Sumas, partly because the agents were busy with people trying to explain why they should get to keep the pot they'd smuggled into Canada and were now trying to get back into the US.
Trailer: 2014 Escape 21...our EscapePod; 2013 Toyota Tundra 5.7L
We were lucky to get thru in about 10 minutes. We bought the trailer in aldergrove bc and took it to Escape industries to get a couple of options added, they looked over the paperwork we prepared and off we headed to sumas. It went very smoothly at the border