It is possible to replace just one leg of your 5th wheel landing gear. You will just need to confirm if it is the lead/driver leg or the follow leg. Usually the lead leg is on the driver side and the follow leg is on the passenger side.
You'll also want to try to find a brand name and part number on the damaged leg. You want to match up the leg dimensions and the spacing of the mounting brackets as shown in the linked diagram.
Most likely your trailer has Lippert landing gear so for a... view full answer...