Border Crossing and Toronto?

We are planning a trip to Maine and it is shorter for us to travel through Canada to get there. We will be hauling our RZRs but should not unload them in Canada. We plan to cross at Sarnia. What should we expect at Customs on both Canada and US sides? I will have our travel itinerary with us and should be in Canada for about 16 hours.
How bad is traveling through Toronto late on a Saturday morning? I will have a 40′ motorhome pulling a 20′ trailer and it’s the busiest Interstate in the world. Some friends traveling with us will have large 5th wheel toy haulers also.