we just rode the 407, a round trip from Hwy 410 in Brampton ,west to Burlington. We do not have a transponder(don't make enough trips on it to warrant having one). We went at 6:30 am on a saturday and came back at 4pm the same day. Pickup truck in our signature-no trailer- cost was $30.00. Pricey you bet, way less traffic-yes.
You won't need a transponder for just one or two rides on the 407. if you use it at all. There is always the QEW, and the 401. Both lots of traffic, but free. The 402 out of Sarnia is a very good road, has been redone completely.
Your trailer has a rear lic plate also, they will go by that.
Hope you find a great route to take. Remember, fridays and Late sundays, the traffic on the 400 is bumper to bumper, its the route the city folks take to "cottage country".
Just plan well and have a backup, and bring your patience and lots of money. Fuel is pricey! Dona