You are right- stopping on a very steep grade would be nearly impossible without brakes on the trailer also. But realistically, how steep of a grade are you go

You are right- stopping on a very steep grade would be nearly impossible without brakes on the trailer also. But realistically, how steep of a grade are you going to want to tow 15 tons up with pedal power? You're not going to be moving much at all. Towing that much on level ground would be a separate stunt from doing his trip to the baltic. Given his top speeds are likely to be close to slug ground speed levels and on perfectly level ground, braking with just the tow vehicle would be fine still.