https://www.cbp.gov/trade/basic-import-export
https://www.riv.ca/
Make sure you have all paperwork when you get to the border.
Call border directly to see what is needed if you cant find the info but you WILL need bill of sale, title or deed of trailer if issued in home state, copy of ad from internet, and any other information that maybe relevant. Again call and ask. Sometimes they require that the paperwork be at the border 3 days in advance of you crossing but that is usually only for motor vehicles. You will need to stop at US border services on way out to verify that trailer can be exported.
Once you get back to Canadian customs then all paperwork required, pay your RIV fee (~$200), federal taxes and get paperwork to get it inspected. Once home, get trailer inspected at Canadian tire (free) then go get it licensed and pay provincial taxes.
Do your homework before you go and make sure you have all paperwork or you'll be making a second trip.
Marpet
P.S. I've imported 5 boats and 7 campers, but not in the last 5 years so the rules may have changed. Call border services in US and Canada if you still have questions.