Another vote for Google Maps.
Looking at the Il to Vt route on GM. It gives 3 'top' choices, based on 'fastest'.
1 - does go through Canada. But looking at the Montreal segment up close (including Streetview) I see that it takes an expressway beltway that stays well outside the city proper. I'd expect heavier traffic as it skirts Chicago and Toronto.
2 - this alternative stays in the US, but skirts Cleveland and Buffalo, and uses tollways across Indiana and Ohio.
3 - crosses PA, and drives up the Hudson valley.
Specifying that you have trailer or monster RV isn't going to changes those choices. They stay on expressways where possible. Apart from traffic around cities, tolls, and bumpy pavement, those are going to be the fastest and easiest roads to drive in an RV.
I could suggest some alternatives:
I88 is the closest freeway to Fulton, and I294 is the shortest way from there to I80. But it gets well into Chicago traffic. So crossing over on I39 would avoid that traffic.
As discussed in many threads, it's next to impossible to avoid the traffic on I80/94 across the bottom side of Chicago.
If time wasn't important, and scenery valuable, I might drive into Wisconsin, and take US2 across the UP, and then take a northern Ontario route over to Ottawa.
I86 is an alternative to I90 or I80 across NY or PA. That has more curves and hills than I90.
And closer to VT, there's the question of when to leave I87. If you get of the freeway too soon, you might have lots of driving on narrower roads, with the possibility of low bridges, and short grades.