You don't say where you are or where you are going.
If there is a Flying J w/ RV lane, or a state campground on the way, you should be able to get set up.
If you top off the potable and really empty the waste before arrival at the stop, you should be able to make 5 days with proper conservation levels. It won't be easy, but if you watch everything like a hawk, you can make it happen.
ON EDIT: I just got to the other post that says "Me and the cat". The cat won't waste water and that leaves you alone and you should be able to do five days on the tanks.
Matt