I'm not very familiar with the Ontario end of things so I can't comment there.

You'll probably be all right on the TransCanada from the Ontario border to Winnipeg. It's a tough call in that area, you don't really have many options. But be prepared for traffic and headwinds.

After Winnipeg, get off the TransCanada. I'd recommend dropping south to Hwy 2 ... it's less busy. When you get to the intersection of Hwy 2 and Hwy 5 (Glenboro), you can turn north and camp in the Spruce Woods Provincial Park. Rejoin Hwy 2 and follow it as it turns to Hwy 13 in Saskatchewan.

You could drop further south if you want, there are several decent hwy options in Manitoba, and as long as you're off the TransCanada, they are reasonably quiet.

You can rejoin the TransCanada in Saskatchewan if you want, but personally, I'd be inclined to stick with Hwy 13 until you get south of Swift Current ... unless, of course, you want to see Regina and Moose Jaw.

No matter what you choose, it will be flat as a pancake and likely very windy. You're riding into the prevailing wind ... but you might get lucky and get easterly breezes. However if you do get an easterly, be prepared for late afternoon and evening storms.

Staying south should be all right as a direct route, through Lethbridge, but it won't be particularly inspiring until you get closer to the mountains. And when you do, be prepared for climbing!!