For our 62 TW, we used two of the thin tanks that Vintage Trailer makes to go between the floorboards and the belly pan. (The tanks are pretty small, and we thought that one wouldn't give enough capacity.)
So far, this works well. If you try this trick with two tanks, remember to connect the two with *two* pipes: one as close as possible to the bottom edge, and one at the top to let air equalize. Even so, we find that we can empty the tanks better if we use our tongue jack to raise the front end a little bit when draining.
Here it is: Gray Water Tank