Because parking with Parkmobile is based on your zone, license plate number, and space number (where applicable), you can pay to park your trailer (if allowed in your parking area) by first adding the trailer's license plate number to your account, and then starting two separate parking transactions—one transaction for each license plate. If you are parking in an area that allows you to stop your session early (See "Can I stop a parking session?"), remember to stop both parking transactions before you leave.
Note that not all parking areas follow the same rules and regulations. You must be aware of the laws in your area to ensure you are allowed to park a trailer in your particular parking area.