It's still not clear to me. The language used so far does not clarify whether the entire slideout frame and construction is OK/normal/sound/without issues.
If it is, I would simply take the sofa supports apart(take sofa out), and relace the substandard/split board with a perfect one. It's my way of fixing things. And, I would first drill a correctly sized pilot hole before screwing-in any lag bolt.
I hope that the slide proper is OK. It's no job for the inexperienced. Of course, if you tackle the slideout fix, and succeed, you will gain slideout experience.