La Smala,
Based on the experiences of others I know who have done this, one cannot rely on any information provided by a seller in the U.S on the importation procedure of a vehicle into Canada.
In order to do it correctly you have two options:
Call several Customs Brokers to identify which one would be interested in taking your importation case. Not all Customs Brokers will take on this type of work, so it may require several calls. Being In Montreal, you may wish to visit the border crossing at Champlain Hwy 15/ I 87. The Customs offices there know which brokers can help you and the brokers offices are right there at the border.
Do it yourself. This would be my personal preference, and is worth the research and preparation.
First visit these sites:
http://www.cbsa-asfc.gc.ca/travel/bringing_goods/motor_vehicles/menu-e.html
http://www.tc.gc.ca/roadsafety/importation/impusae.htm
Call the contact numbers to get precise information on the make and model you wish to import from the Registrar of Vehicles and find out what modifications need to be done after you've cleared Customs.
If the RV is new, and you're ordering from the factory, you could order it with a Canadian Package. If it's used, it may already have a Canada package for lights and safety.
If the vehicle is manufactured in Canada or the U.S. there is no duty, but there is excise tax and GST. Excise Tax id $100 for the air conditioning, and an additional excise tax for vehicles over 2000+ KG. Check with Customs for the sliding scale.
Once you get all your information and paperwork, the Customs office where you plan to cross needs 3 working days notice of your arrival so they can prepare for your crossing. You then have a certain amount of time to get the vehicle inspected by the Registrar of Vehicles to get your plates.
Here are a useful phone number:
(888) 848-8240 vehicles that qualify for import
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What coach do you plan to import, from where and at what time of year?