We can't afford outside help, but I'm at my wits end.

Get her to the hospital and then get a social worker to help you apply for Medicaid for her so she can enter a nursing home.

Call aps and self report the situation.

Your mom has long ago needed a nursing home as you and your dad are not giving her the care she needs to have. I’m guessing she has sores all over her skin?

I would start with calling APS(adult protective services)and report the situation. They will come out and do an assessment and take things from there, and if need be, take over your moms care.
And obviously if money is an issue, Medicaid can be applied for, so mom can be placed in the appropriate facility.
Please call them first thing Monday morning.

Do you live with your parents because you're their caregiver? Or for financial reasons? I'm only asking because if you are able to leave/move out then this is one strategy. You move out, stop helping and report them to APS. They will take it from there -- but on their own timeline. If you cannot move out, then call social services for your county and get an assessment for some in-home services. I'm assuming you're not the PoA for your Mom so when they come you may also be able to discuss guardianship if she is refusing care and is a danger to herself. Where is your Dad in all of this? Is there no money to hire an in-home aid to help get her showered and perform other tasks?