Ah, good call. I found the line that needs to be replaced. I have added screenshots for placement reference.
This line:
<trailers />
need to be replaced with these lines:
<trailers>
<Item>boattrailer</Item>
<Item>trailersmall</Item>
</trailers>
Those lines were copied from the Bison's meta as the Bison is one of the vehicle in the base game that can tow trailers.