Tesla Motors does not recommend that you ever tow a Roadster in the manner described: with the Roadster in Tow Mode and the rear wheels free rolling on the ground.
There are many reasons why this is not a good idea, but at a basic level, law requires that the rolling wheels of a towed car be mechanically disconnected from the drivetrain. (In a normal RWD car this is accomplished by disconnecting the driveshaft.) Even in Tow Mode, the Roadsters rear wheels are permanently connected to the gearbox and motor.
Tow Mode is designed specifically to allow a user to slowly load the Roadster onto a trailer.