How do I "force" my parents into assisted living?

You can try to enlist the aid of the doctor. You also could ask Social Services to do a welfare check on them. From the sound of things, they aren't safe the way the are, so this would give you official backing to get them moved. They'll complain, but if they are told by officials that they need to go to assisted living or something that could make it a lot easier. Good luck. You are right - they need help.
Carol

Do you know their doctor? Can you arrange to go on an appointment with them, and alert the doc to the situation, so she or he can be the bad guy here? When my dad heard his doc say he could no longer live alone, that got his attention.
Do all the research ahead of time so you can present some options with knowledge. Be sure you understand their financial situation so you know whether you can get them into a continuing care situation ( where they will get more care as they need it and not be kicked out). Good luck!

Your post brings back memories - getting the doctor involved is a great route to take - as Jane said let doc be the bad guy. I had similar problems with my parents and it was Elder Services in their town that helped me. They are a great resource and very supportive - which is what you are going to need in this situation. My parents resisted help as well and protective services had to be called in. Hope you can resolve this and wish you well. Take care.

Justin, well trying to reason with your mother is not an option since she has dementia, and those days of logical thinking are done. But your dad is another thing. Can you take your dad aside and tell him you're worried. Tell him that you're worried if he were to take a worse turn in his health and have to be hospitalized, you don't know what you'd do with mom. Tell him that you don't know what you'd do if he weren't able to help you with your mother since her mental reasoning has been compromised. The only thing I can suggest is to appeal to your dad's helping you with your mother. Maybe if he sees that by being so self-sufficient he's going to cause you grief, he'll start thinking about the bigger picture. Other than that, your only option is to have a doctor/doctors be the bad guys and force them to move. Good luck.

If you have DPOA and MPOA for them you can do it if you plan it out and have a place for them to move into. It sounds like they are at the point they are not competent or cognitive to make decisions, so use your DPOA and move them. And have an agreed upon understanding with whatever family that could have a say in the move that it needs to be done.

If there is not DPOA, then it's sticky as you will need to get Guardianship or Conservatorship over them. As one who has been executrix twice and spent buckets of hours in probate court (where G/C stuff is heard), you need to get an G/C experienced attorney if you have to go that route. G/C isn't complicated per se but has a lot of sequential legal items that can be maddening for non attorneys.