Use gray to flush black

When our gray water tank starts to get full, we open the valve and drain some gray water into a bucket and then pour the bucket into the toilet. Both the gray and black tanks are 25 gallon, but we generate about 5 gallons of gray water a day and maybe one or two of black water. So this "bucket brigade" helps extend the time that we can boondock, and it also dilutes the black water.

A pump would be very useful, but I am not sure how to plumb it safely.