What is the best approach for filing a claim, grouping multiple issues (GERD-Hiatal Hernia and Sleep Apnea) or filing separate claims?

Gerd and Hiatel Hernia are rated together because of a regulatory requirement. Pyramiding is what everyone refers to. Its explained in 38 CFR 4.14. Also your sleep apnea will be evaluated with any of several respiratory disorders, asthma, bronchitis, etc. Essentially they rate them separate and then award you the most advantageous evaluation.

Sleep apnea requires evidence in service such as a sleep study. You may have a shot at secondary service connection if you have a SC condition like TBI (for Central Sleep Apnea), or upper respiratory condition like sinusitis (for obstructive sleep apnea). Both of those are known conditions that can cause sleep apnea. Of course the MOST common cause in older men is deconditioned neck muscles. That makes it pretty difficult for a doctor to write a plausable statement about a guy that has been out for 40 years, and try to link it to a 1.5 year stent in Army, for example.

I would talk to Veterans Service officer, he or she could give you more info about filing and how to go about it.

Whatever you want to claim, you need to have proof that it is service connected or you will get nothing for them.