Every Motorized Vehicle has to have it's own Liability Insurance policy!Depending on You , a more comprehensive policy may or should be purchased to cover damage due to Collision Etc.
Trailers are an entirely different category and are usually included in the Liability portion of the Towing Vehicle.
A separate " All inclusive policy", may also be purchased to cover said Trailer and it's contents. Remember that a Trailer that is not attached to a tow vehicle is not covered by the Tow Vehicles Liability Insurance.
Another little item that isn't often discussed concerns the actual limit placed by these Insurance Companies on the Cost of Repairs!
In Mexico repair costs are considerably lower than in the USA or Canada. As a result Mexican Insurance Companies often limit the Labor Rate to around $40 per Hour. This is too low to cover repairs done North of the Border.
It is up to You to Ask the Insurance Agent about the optional Higher Labor cost "Add On"!
Note: There are really only 3 insurance Companies that offer coverage for Foreign Owned Vehicles in Mexico. Their Rates are very Competitive and the real differences that Agents quote to "SellInsurance for Less" are just optional items such as Labor Limitations , deductibles and Legal representation!