You say
We want her to use the equity in her home to support herself and think the best way would be a RM
but then you immediately continue with:
I'm just not sure about the fees, how RMs work or if there is a better option.
So why would you think that? You don't explain what lead you to that conclusion, especially given your admittedly zero knowledge about how RMs actually work or how much they cost.
The first thing I'd suggest is budgeting. Understanding where and why the money disappears. If she's living beyond her means - is the means the problem, or the living? Can she cut expenses? Can she reduce costs?
Once you're done with budgeting, the next step is the balancing. You know the expenses, you know the income, you know the difference. You need to cover the difference. How much is it? If it's a lot, maybe go back to the first step and see how else she can reduce costs. Move in with some of the kids maybe? Move to a lower COL area? If it's not a lot - consider how to cover that difference. Help from the children? HELOC? Second mortgage? Reverse mortgage? Each has pros and cons.
Generally, when it comes to the various ways to extract equity from real estate, reverse mortgages would be the more expensive way. The reverse mortgage companies assume the senior person utilizing their services doesn't understand, doesn't know, or doesn't care about their fees and tuck in a lot of fees and costs into the mortgage. A regular HELOC, if available, would be by far a more efficient way to extract equity.
In any case, there's a considerable risk of exhausting the equity credit limit before passing so I'd strongly advise to consider other options first and resort to equity loans (of any kind) as the last resort. You don't want her to end up not only with expenses exceeding the income, but also homeless.