While lowering my awning a couple weeks ago, I noticed one of the sliding brackets had a good size crack in the aluminum casting. I check the bracket on the other side, and it too was just starting to crack.
I have no idea how long they've been cracked, But I guarantee, I'll be looking at them from now on every time I lower or retract the awning.
I ordered two new brackets and replaced them both. Rather than use the rivet to secure the bracket to the rail, I used a stainless bolt and a self locking nut.
I wrote up the complete repair on my blog.
Awning sliding bracket support bracket cracked on both sides | 2001 Coachmen Mirada 300QB