I'm not sure if a tow bar would equate to a "safety device" or not.
To me, a safety device would be the bumpers, backup/brake lights, etc. A hitch, tow bar, etc. are not used to provide safety to the vehicle, but to enable connecting the two vehicles, so would not be considered "safety device". But, might be worth it to ask highway patrol on that matter.
As to whether a B is over or under 20 ft, it will depend on what you get. A sprinter could be 20-21ft. A simple conversion van could be as short as 16 ft.
What you get will really depend upon the amenities you must have. Do you want a bathroom? Porta potty? Do you want an indoor kitchen? How will you use the B? Just as a place to sleep on the rare night you stay away from the DP? Or do you plan on using it as a substitution for your DP when you can't take that?