Living in southern Ontario we have travelled this route quite a bit.
The busiest times going through Toronto is your typical rush hour (hours) time.
From 6:30 to 8:30 am. and from 4:00 to 6:00 pm.
The QEW and the 401 being the major routes into Toronto could be heavy at any time but take your time, relax and you will be fine. Give yourself some extra "tourist" time. You may want to stop along the way from Niagra to Toronto and visit the many vineyards.
Travelling through Montreal at anytime during the day will be hectic. Montreal is a beautiful place, the people are great but traffic, huge headache. I would plan to go through Montreal very early in the morning if you are concerned. The only thing is it will be dark and you would be missing a lot of the sites of Montreal. Remember, all signs are in French only so since you are not going until next fall you have a bit of time to do homework. Try getting a driver handbook online to review signs.
Old Quebec City is absolutely somewhere everyone traveling east needs to visit. Driving in the old city will be very hard with the RV. I suggest that you unhook at your campsite or take a taxi in. Parking would be impossible in my mind. If you were going to stay in a hotel/motel along your trip, this might be the place to. There are quite a few just off the major highway and within reasonable taxi cost. Book anything, campsites, hotels etc. around Quebec City as soon as you can and expect this area to be your most expensive on your trip.
Anywhere in Quebec we have found people to be very helpful with language. Most people living in the Montreal area are bilingual. The only issue with language we have had over the years was in rural northern Quebec. It was not that people were being rude, they just did not know any English. You will find many french speaking areas in eastern Ontario and northern New Brunswick.
Like you are doing, homework is the key. I am sure your trip will be a great one. Are you planning on doing all the Atlantic provinces and how much time do you have scheduled?