Sliding Mirror Door in Rv, Bottom Track Guide broke

I'm afraid that plastic piece isn't available by itself, so you will need an all new guide assembly.

My recommendation would be either the 23-215 or 23-026. Note that the major difference between them is that the 23-215 has a flat horizontal mounting bracket, whereas the 23-026 has more of a hook shape to it. Look them over and see what you think.

Alternate view of 23-215

Detail of 23-026

Profile view of 23-026