That's almost certainly correct. The US Army would have it's greatest presence in the more populated areas. I the north of Canada, beyond the main east-west rail lines, they could likely have only battalion and company size units stationed in larger towns or villages. In fact, that could likely be the impetus for the development of airborne or air transportable units, to enable them to rapidly move reinforcements around.
If I was to guess the US held garrisons in almost every town along rail lines and then major hubs where HQs would be.
General Custer complained that he didn't have a lot of troops in Canada, with his exact comment being he had less troops under his command then he would have lost in an average medium sized battle of the Great War. I don't doubt that after the rebellion the US Army was allowed to beef up its detachments but still overall isn't huge.
Going along with my guess the US Occupation of Canada mildly looked like this
Occupation HQ: Winnipeg
(While likely done for literary reasons Winnipeg was now more central to Canada with the Maritimes and Canada on the other side of the Republic of Quebec (also notable that Newfoundland and the Maritime Provinces were the only areas not explicitly mentioned to have rebelled during the rebellion of the 1920s or 40s)
1st Occupation Brigade - Covers Ontario
HQ: London
Major Garrisons: Berlin, Toronto, Ottawa, Kingston, Thunder Bay, Windsor, Niagara
2nd Occupation Brigare - Covers the Praries
HQ: Winnipeg
Major Garrisons: Regina, Saskatoon, Edmonton, Calgary
3rd Occupation Brigade - Covers mainland BC
HQ: Kamloops
Major Garrisons: Banff, Vancouver, Surrey
Minor Garrison forces would likely be on Vancouver Island but don't he surprised by the Navy is doing that and are only really occupying Victoria because of the Naval Base.
In addition there would be major Naval and Marine Garrisons in St. John's and Halifax while the Republic of Quebec probably assisted with the Occupation of Northern New Brunswick (while not on the map wouldn't be surprised of the Acadian north of the province and Labrador was given to Quebec after the war. To annex)