I commend you, K., both for caring for your parents in your home and for caring about your parents. I read so many posts about horrible parents and even more horrible in-laws that, in spite of the problems, your post is a breath of fresh air!
You have some good answers already. Nutrition is very important. So is any sort of exercise they can do. Do they go outside much? Get them out of the house! With you, of course. (When my mama was living, I couldn't do much for her because I lived several hundred miles away, but when I was able to visit I'd get her into the car and we'd just go driving around. She would sometimes say that it was nice to see the world outside her four walls - even if we were just going to the grocery store!)
You're in touch with your parents' doctors. Stay in touch. Don't be afraid to ask them the questions you're asking here.
Think about how to help your folks and direct them in the right ways without seeming to "manage" them.
As has been said, let your parents know you love them! Every day, every hour, let them know how valuable they are. Don't assume they know.
Depression is RAMPANT among older people. If you use your imagination and put yourself in their shoes, you can start to understand why. And physical exhaustion can be a component of depression. So the conversation you had - that might have been the depression speaking, not really your mom.
Is there some sort of senior center near you? Perhaps you can get your folks to investigate some of the activities (you go with them), so they might find some new interests or dust off some old ones... AND meet other people their age. Friends can help a whole lot. And it will help you, too, because you'll be able to meet the people who run the senior center, and you can pick their brains.
I'm older than your parents are, K., and I hope that, when I need it, my children will be there for me the way you are for your mother and dad.