You do not really need to use the unit just to top off. That was what you could do if you wanted to plum it into the sump. Personnally I don't like that.
What I would suggest is get your place where you want to have it set up. If it is next to the washer, then get one of those "Y" things from the hardware store.Attach it to the cold water spiket. One side is for your washer, the other for you RO. then when you want to use it, just turn the flow thing on the conector to go to your RO unit. I run my RO into a 32 gallon Brut trash can, That would work for you. You can put a maxi-jet powerhead in the bottom of the brut container to pump your water in to what ever contain is easier to handle. See the pictures to give you idea.
That way if you only need 1 gallon for your topoffs then just get a gallon out. You don't need to drag around 5 gallon buckets for no reason.
For water changes. If you use a 5 gallon bucket, then just fill her up and do what you need to do.
I would suggest when you are not needing to get water out of the brut, just take the hose off the powerhead so the water can circulate. When the Brute is full just switch the water at the "Y" back to the washer.