I am not sure that the answers given here are correct.
I am assuming the trailers are fitted with DFT plates (VTG6) and are therefore in scope of Operator Licencing. To the best of my knowledge there are no exemptions within O licencing for trailers. Exemption examples would be Road Rollers or vehicles only going for testing.
Given the trailers are “normally” kept on this customers site working on the basis that there is a rotation system in place, I personally think you need to add the customers premises as an operating center for at least one trailer. Additionally i would be tempted to add a goods vehicle. Assuming your current center has two trailers listed you may want to consider reducing that by one.