What do I do when there is no where else to turn?

Hello Fawnby, after reading what you wrote. Actually, there are a few things you can do. !. Go online and type in State Attorney Complaint, all States have Elder Laws that are govern by the State Attorney General, and after you are at the end of the complaint, ask the State AG office to forward your complaint to the proper authority. Another option is consulting a fraud Attorney, they will know the steps that you can take concerning his wife using your father as her meal ticket. Concerning your father's Dementia, i would choose a time that you can pick your dad up, take him to the emergency department at a hospital near you, and when they ask why you brought him in, tell them he is refusing to care for himself, this is cause in any State, any Hospital to admit him. Then the next morning contact that same Hospital (Social Services) set up an in-person meeting, then tell the Hospital Social Services staff member in the meeting how his wife is taking his money and causing him to be in an unsafe environment without food, without housing, without medical care placing your father at (Risk of Death). By any States Law, a hospital is by Law obligated to report it to authorities. When you file your Complaint with the Attorney General office online, file your complaint under fraud. The Attorney General office uses your State Police to investigate it. The reason the SSI office could not help you is because the SSI office does not have any authority to do anything. The reason APS cannot do anything is because they need proof or cooperation.
Now, while your dad is in the Hospital, go to that hospital medical records department and tell them you need a copy of your dad's entire medical record they have a form that must be signed, ask them if the nurse brought your dad to their office if your dad signed the medical records release form if you could get a copy of the Entire medical records. After that meeting pull his nurse aside and ask them if they would kindly escort your dad in a wheelchair to the medical records office so he can sign the form.
Keep mentioning your dad's " Risk of Life " to everyone you talk to that I have mentioned. Once you have the copies of his medical records search through them for any Provider's diagnosis of the Dementia. Once you have that document, you can prove that your dad has Dementia, and through the Attorney filing in Court for Bank account records then the Attorney can prove in Court the Fraud, and the Court will end his wife's control of any money. Then plus his wife the Court may order that she return to your dad any money that she has taken, the bank account records will show how much, and your Attorney will obtain the exact amount from the bank records. Make sure you ask the Attorney to have the Court to order/revoke any of his wife's handling of any of your dad's money. Then ask your Attorney to ask the Court to award you as Sole Guardianship for your dad.
It is very possible that his wife may be facing at this point possible jail time if it meets the Fraud Laws. Fraud Laws is simple all you have to prove is someone got money from someone (your dad), and that person (his wife) benefit from the money, and the person who got his money taken (your dad) was caused harm (homeless/hungry/penniless) and risk of death from an unsafe environment.
If you have a camera phone, take a picture of what I wrote and please let me know if this helped. Now, you have a lot of people involved and, in your corner, and you are no longer alone. I recommend that you do not say a word of what you are doing to his wife, let the blame fall where it may.
Your dad will thank you, and you are doing this because you will soon be able to help your dad. I hope this helps you