so is 29 ft the cutoff for not towing a toad ?

You must be avid bikers. We did the no toad for a number of years. Years ago we would take bikes to go to town until town was several miles away. We then rented cars at the destination if we wanted to see the sights that were too far away for bikes until we ran into a situation where there weren't any rental companies. And then there was the time the RV needed to be towed and luckily we had a car with us.

You can do as you are planning and 29 ft is a good size for that, but then so was 31.