Black water disposal for permanently parked RV

First off, I know very little about RVs. Background...I live in the country with a septic field and no access to sewer lines. I am considering having my Mother-in-law live in an RV on our property for a year or two. As far as black water disposal is concerned, our septic system is maxed-out with the number of people already living in the house, so disposing of the black water in our septic tank is not an option. I know one option would be to drive the RV weekly (?) somewhere to dispose of it. I have never owned an RV and would like to know if once a week is realistic for one person living in it. Also, do I have any other options as far as black water disposal?

Thank you,

Mike