My heart breaks for you. I can only say what I'd do. I wouldn't want him to continue for long feeling so awfully sick unless the prognosis is pretty decent. I wouldn't spend that much money myself and I have plenty. It's just that unless the vet thinks there's a pretty good chance of him having a good life, personally, I'd let him go. But that's just me. Everyone views these things differently. And I could not presume to suggest a decision for you. That has to be just the right thing for you and Ralphie. I don't know how old your children are but I wouldn't burden them with thinking about what to do, or helping you make a decision. I don't think kids are equipped to make a sound choice with something like this. It's hard enough for us. I think this has to be the adult's job and choice with some input from your vet.
I had a sick dog (a 4 yr. old Dobe...liver disease) that I spent about $3,000+ on with blood transfusions, ultra sounds, laproscopy, biopsy and ongoing meds, a nutritionist to help with a special diet that I made at home with a kazillion supplements. There were blood tests every month or two to check his liver enzymes. But he was pretty dang comfortable and enjoying life once we got things under control. Then he got stomach cancer all over his stomach and that was that. I had to let him go right then and there when it was discovered, suddenly. So there was no choice at that point.
I hope you don't reach for that self punishing guilt thing. You can not go back. And I have no doubt that your Ralphie has felt loved and happy being with his family. Dogs don't have to have bazillions of things to do. They're evolved as domestic animals to crave being with their humans, along side them. That's more than a lot of dogs get. Just what you're doing for him now shows that he's had a good home with good care. Don't do that to yourself...beat yourself up and look back. So see what the vet thinks in the next little while and do what you think is best, most loving thing for Ralphie. Be careful the vet doesn't string you along, trying too many different tests and procedures, racking up the bill if the prognosis for a long and comfortable life is doubtful. Maybe he's not that kind of vet. I just know there are some that will milk us for everything they can. I've had both kinds.