Assuming your Burro
has the original exterior finish, it would be gelcoat. This is a type of polyester resin that is sprayed into the mold when the trailer is built.
In a nutshell, if it is still glossy, you can simply wash and wax it (look for a good quality wax* that contains UV inhibitors, as UV will damage the gelcoat). If it is slightly to very chalky and dull, you can "compound" or wet sand to slough off the oxidized gelcoat and get down to fresh gelcoat (within limits as the gelcoat is not very thick).
If your Burro has been painted, then it would depend on the type of paint
, as to how you would maintain it.
Until we know more specifics, this is an overview.
*I like Collinite 885 or 3M's paste wax.