You have to read both policies to see how each is worded.
Many are worded 'while your trailer is hitched to your TV'.
You might to have collision on the TV coverage to cover an auto accident which is damage caused by the movement of a vehicle.
Liability might only cover you if you damage the property of 3rd party. If you hit someone's property with that trailer and damaged their property, that would be covered under the TV liability but not the collision of the awning on the trailer.
Its all in how the policies are worded.