I think you have conflated three issues, but each of them is separate. [This is the problem with doing business with family, lovers, or close friends.]
Issue 1: Your girlfriend is moving in with you and owns furniture that she doesn’t want to sell, and evidently you don’t want in your home.
Issue 2: There is furniture that needs to be stored while your girlfriend lives with you.
Issue 3: There is a property that you own which could create some income using AirBNB.
Issue 1 is completely and totally separate. She is moving in. You are not willing to add her things to the home to make it a home for both of you. The two of you will come up with an agreement about shared costs, etc. This has nothing to do with Issue 3.
Issue 2 is a little more murky. As a way of saving money, she would like to furnish the AirBNB property with her furniture. It is hard to tell if this property will just sit empty if you don’t put it on AirBNB in which case, storing the furniture there makes perfect sense. If you would normally rent it to LTR folks, then it might not make sense. This is both personal and business.
Issue 3 stands alone. Ignoring the furniture for a moment, this is all about business. If you had a property manager, they would expect 25-30%. For that amount of money, they would do all the communicating, deal with problems [maybe], arrange the check-in, arrange the cleaning folks, and order supplies, etc. They will not clean the place. They might not even check that the cleaning was done to your satisfaction. Depending on the size of the space and the cost of living in your area, the cleaning can run between $15-$35/hr, usually bid as a flat fee. It is really possible that you are at 50% now. Property managers and cleaning folks do not pay the mortgage, utilities, purchase new sheets, replace damaged fry pans, or contribute in any way to the cost of owning and operating the business.
I suppose you could charge the aforementioned girlfriend $100 to “store” her furniture in your house, but then you would be responsible for the wear and tear done by people renting your home, or you could reduce the fee so that any wear and tear was part of the rent.
We can help with Issue 3. How much would it cost if you had a really good property manager who really cares about your property, and a reliable and terrific cleaner? What percentage of the income is that? Without you knowing this, there is no way for you to know if your girlfriend is asking for an amount that makes sense.