It will depend on what kind of RV.
First, you are importing it. There is a general exemption from certifications and duty for vehicles casually imported by visitors, if the vehicle will be in the U.S. for a year or less. But that is not what you are doing, you are importing it for sale. That makes you a commercial visitor, and the importation becomes a business.
Motor vehicles in the U.S. are subject to DOT safety and EPA emissions certification. While U.S., Canada and Mexico are more or less a single market for automobiles, and there is no great difference in standards, something sold in only one country may not have certification documented for the other countries.
Non-motorized RVs won't have EPA certification, but there is still a certification procedure for DOT safety requirements. Here, one would hope that the customs agent clearing the importation would have the good sense to waive EPA certification.
Then there is customs. Duty might range from 2.5% to 25% depending on how the Customs Bureau chooses to classify it. It is possible there may be no duty on a trailer, but it might end up being taxed as a car.
If you are a U.S. citizen or permanent resident, there are procedures (including duty exemptions) for importing a vehicle bought in Canada. These assume a physical presence in Canada for the purchase, and clearing the customs and certification at the point the border is crossed.
Customs information page.If you do not qualify as a citizen or resident, you cannot import a motor vehicle for sale without an import license.
Bringing it in as a casual visitor, then selling to a qualified buyer, can create major paperwork headaches for the buyer. This could make it difficult to sell.