Yes, and no. If you have chronic nasal congestion, then treating your deviated nasal septum can definitely help you to breathe better through your nose, and there's even a small chance that it may help your sleep breathing problem. Take a look at my upper airway resistance syndrome article on on my blogs on this site. You can stop breathing and wake up 20 times per hour and not have any apneas whatsoever. This is a flaw in the sleep study scoring system.
So for the sake of breathing better through your nose, it's probably worthwhile to do something definitive for your deviated septum. However, if you don't have any nasal congestion, then al this is a moot point. After the nose is address (if necessary), your problem is more likely due to your tongue. This is why you can't or prefer not to sleep on your back. The article on UARS explains this in great detail. It also explains your chronic fatigue.