I know what you are going through. I just lost a heart dog three months ago, a month and a half before his sixteenth birthday and the void is still as big as ever. His last year he was on medications to make him more comfortable and he never lost his will to live, even showing the pup the ropes and becoming his best friend.Thanks neighbor. I didn't realize you were in AZ but after looking at your avatar with the saguaro cactus I should've figured it out - "duh" on my part.
Just wanted to mention the vet may or may not suggest putting him down. My experience with the good vets at Arrowhead Animal Hospital has been they'll suggest courses of action and let you make the decision, and even if they lean toward euthanasia, it's totally up to you if and when to do it. I had two dogs put down there and in both cases they were very supportive of my decision. The first was a Springer Spaniel (Opie-my best hunting buddy) that had stomach cancer just before he was 5 years old. We found out on my birthday of course - these things never seem to happen at an appropriate time. Anyway the vet suggested putting him down but also told me the alternate course of surgery and chemo which only had a small chance of success and virtually no chance of a decent life afterward. So of course I fell apart and my wife had to do the honors. I'm glad she was there or the vet wouldn't have known what I wanted done. Went to the same place for our cat and had a similar experience, but our cat was 21 years old and had only been having problems finding the litter box about a week before I took him. And then with Chester the vet suggested we run some blood tests to see if he had cancer, which the vet suspected in his throat, but I was the one to recommend putting him down. The vet said I know my dog best so he agreed that was probably the best course of action. I think the vet was more sad than I was. Anyway I'm going the long way around the barn but the point I wanted to make is not to fear the vet because you're in complete control of your decision. He/she will lay out your options and he might make a recommendation but it's totally up to you. And with Chester, he had arthritis pretty bad toward the end but the vet put him on a pain med as needed and Chester really perked up for a good 6 months before the end. Your dog sounds like he might benefit from a similar treatment assuming no other major issues, so I'd at least take him to the vet for that issue and you can feel free to discuss everything else while you're there. That way you likely won't have the pressure of having to make a major decision just right yet.
Two things that might help. If you have a trusted vet find out what is going on (without bankrupting yourself. Some vets just want to do test after test, so you need a vet you can trust.) then ask her if it were her beloved dog, what would she do. I've asked that question several times and have gotten honest answers.
And the other thing...many will think it quackery but if you can find a good (operative word being "good") animal communicator, have them talk to your pet to get his point of view. He may be ready to leave or he may not. I do believe there are many charletons, but some are spot on. We had one dog I had had to several vets and dog chiropractors (He wasn't old.) Finally I got hold of a communicator who was fantastic (She has since died unfortunately.) and he told her exactly where it hurt and what the problem was. Using that information the vet was able to get him on the road to recovery.
Best of luck to you. I know this isn't an easy time.