Here is my very low-tech solution -- when the gray tank is nearly full (after five days of camping), we fill an ordinary bucket (not too full!!) with gray water and then carefully carry it into the bathroom and dump it down the toilet.
Since our black tank has way too much capacity (25 gal, same as the gray tank), we "borrow" the capacity of the black tank.
This method is tedious and slow -- the bucket only holds two gallons without risk of spilling. And it has to be done carefully -- don't want gray water on the floor of the bathroom!
But the advantage is that this method works without any modification of the plumbing.
