The fiberglass piece along the rear is sometimes referred to as a BEZEL.
Here's a shot from the time when we were decommissioning the trailer in 2006.
This is the aluminum casting of the word ARGOSY. It is all one piece; you won't be able to find "letters".
This is my neat little trailer when finished:
Refinishing those pieces took hours and hours. I had to carefully mask and spray them to get the effect I wanted.
And since the front and rear of the trailer provide different backgrounds for the logo I had to make each a reverse mirror image of the other.
You might be able to find replacements in a RV salvage yard. There is a famous one in Missouri, the name forgotten just now but some one here will chime in.
I bought a window from a member in New Hampshire who was parting out an ARGOSY. Maybe I can find his name for you.