Are you sure it says tow car and not towed trailer? They are not recommended on the back of trailers due to lightening of the tongue wt along with the vibrations which can cause them to break.
Motorcycles or bicycles? Bicycles aren't going to lighten the tongue weight. Most travel trailers don't have shocks so you can get pretty violent bouncing clear at the back - not good for racks or bikes. Not true with a towed car.