Buzz,
Has anyone recently traveled through Canada with a firearm? Yes. Me. I think I posted that information before, but owners of FWC/ATC campers need to know the facts.
Canadian law calls for a minimum barrel length of 470 mm (18.5 inches). Your 18 inch barrel is too short. I had the same problem. My home defense shotgun (pictured) has an 18 inch smooth bore barrel, which is legal in the United States but is illegal in Canada. Thus, I purchased a new 21 inch fully rifled barrel and installed it for the trip to Canada. The different barrels drop in and can be easily switched. I left the illegal in Canada 18 inch barrel at home in the US. I took the now rifled gun with its 21 inch barrel to Canada. Pictures are attached of the same gun with different barrels, shorter barrel not legal in Canada and longer rifled barrel legal in Canada. My US defense gun is a smooth bore buckshot scattergun. The Canadian defense gun is a large bore .73 calibre rifle. Both are the identically same firearm with different barrels. By the way, the rifled gun would be more effective against large bears and, hopefully, never to be used.
Canadian authorities gave me no problems with my sporting firearm when I crossed the border in September this year. One of the reasons I attached the pictures is to make the point that my gun is a sporting firearm and not an aggressive looking military style weapon. I was charged $25 to register it for 60 days. I paid with my MasterCard. The border control officer told me to keep the gun out of sight inside a locked container. He specifically told me that my camper was considered a container and to keep it locked. I actually did better than that by cable locking the gun to a secure point under one of the seats. The gun came out only when we camped alone in bear country.
John D