Trans Superior Hwy

Those are good roads and major transportation corridors through Canada. You should have no problems with your rig. Ontario does seem to go on forever when you're driving across it. The scenery is pretty along the way; not generally spectacular vistas, but varied and pleasant.

I don't know that there is a "best time" per se, though I'd probably avoid the winter months. Parts would be very pretty when the leaves are changing, sometime around late September or the first half of October.