You should ask your vet to do blood work and test her stools for worms or parasites.
I don't think she would want to eat if she was suffering but there is something wrong.
You need further test to find out and unfurtinatly that cost money but if you find out whats wrong it may be treatable.
As long as she is still eating and grooming herself, then she hasn't given up on life, which should mean she isn't suffering unduly - that's a rule of thumb that I have used and I find it's true. A cat that's in a lot of pain will just give up, stop their normal behaviors, stop trying to maintain life by eating and drinking and just endure. If something changes to make it obvious, then that's different of course, but otherwise, it's something to go by.
Otherwise, all I can say is trust your instinct. You have known her for a long time, you will probably know when something drastic changes for her.
The rough fur and increased sleeping is perfectly normal for a kitty her age - she's an old girl and she's earned her snooze!
You'll know when it's time her time as she'll stop eating. I agree that stool samples to the vet are needed to be checked.
My Max was 17 and in January he stop eating and drinking and would just walk and walk without stopping and so we knew he was gone as his eyes would be spacey. so he was put to sleep on 9th of January 2008, vet said he had heart problem and thyroid problem and meds for one or other would kill him. It was a very hard choice to make...I so miss him at times.
Enjoy the time left that you have with your cat.
Might be you just need to switch to a better cat food, go to a specialty cat store and ask them.