Elderly Dog Advice

Hi all - just looking for advice and input as this is a difficult situation and would like to get other points of view.

I have a Chow/German Shepherd/Pitbull mix that is 15 years old and 50lbs.   I've had her since she was 6 to 8 weeks old and she's been a great dog. 

We have a typical house and she does have to go down carpeted stairs to the basement to get outside.   We have 2 little kids 3 and 5 years old and a 8 year old cat.  She's never once been aggressive with the kids. She and the cat tolerate each other.

Over the past 2 years or so her mobility and health has declined greatly while she doesn't have any diseases or anything that the vet has found she has the following issues: 

1. Inability to go down stairs - she seems to either be in great pain or has vision issues and trying to go down a typical staircase takes about 5 min, i've now started carrying her down the stairs.  She's also fallen down the stairs several times.
2. Lack of interest in the family - pretty much stays in her bed in the basement almost all hours of the day and sleeps
3. Sleeps all day and all night 
4. Erratic eating - meaning sometimes she eats her food sometimes she doesn't.  She has an automatic feeder that feeds three times a day morning/noon/night sometimes it's all eaten sometimes only a little is eaten.
5. No interest in staying outside or going outside - as soon as she's put outside she wants to come back in, that's all types of weather and all times of day, I've tried putting her on a leash and walking with her in the yard and she has no interest in that either.  
6. No longer enjoys doing the things that she liked to do - such as fetching, going on walks, playing etc....  When she was younger we used to do 6 to 8 mile long walks. Now we're lucky if we can go 100 yards .
7. The vet has diagnosed her with doggy dementia - sometimes she will stare at a wall for 15 to 30 minutes, sometimes she'll walk around the house and look like she doesn't really know what to do or is lost. 
8. Has no control of her bowel movements - according to the vet this is due to her nerves that no longer work along the base of her spine where her tail is, vet says there's no treatment for this and it also effects her tail, it no longer comes up or wags, just kinda hangs there straight.  With the nerve damage poop essentially just falls out whenever, it happens while she's walking and it happens while she's sleeping and she typically just lays in it.  I do my best to try to clean it up as quickly as possible but I'm not joking when I say we have to pick up poop 10 to 15 times a day.
9. She is going deaf - i have to raise my voice fairly loud or clap my hands to get her attention, and she's also going blind and has difficulty seeing.

I have discussed all of this with the vet and other than medication for pain(which hasn't made much of a difference in the past)  they say to just make her as comfortable as possible.  Blood tests don't show any health issues.

I've done 5 of the online assessments about quality of life and they all seem to come out about the same 35 out of 100.  Really I'm concerned about quality of life and if anyone has experienced this before and what steps you've taken.  Any advice would be greatly appreciated.