replace vs repair
Assuming that this rear cap is fiberglass, I would expect that your best and possibly only option is to have it repaired. Speaking from personal experience, you would be amazed that a good bodyshop is able to achieve. Look in your phone book or ask at your local truckstop for the name of a big rig bodyshop. Semis require this type of work on a regular basis ... the cost will be less that you expect.