Insurance companies and their policy terms differ as do state laws.  So it varies within the insurance industry if a towed vehicle, such as a travel trailer is covered or not by a motor vehicle policy. 

In some cases a normal liability insurance policy might cover the trailer while it is being towed but the travel trailer would likely need its own comprehensive or collision coverage when it is unattached to the truck.  In other cases the trailer might need to be listed on the declarations page of your policy to be covered.

Since insurance companies guidelines vary, I would suggest reading over the terms of your comprehensive coverage and contacting your insurance provider to see if the travel trailer is covered for the incident that occurred.