If the total cost is $2500 or less you can just expense it otherwise it is an asset subject to the depreciation rules. You can choose to depreciate, bonus depreciation and/or 179 deduction in any combination that works best for you. A trailer is a 7 year asset for depreciation.
Read up on the choices you have here :
https://www.irs.gov/forms-pubs/about-form-4562
https://www.irs.gov/forms-pubs/about-publication-946
