Installing a gray tank doesn't involved modifying the frame, but it does involve a whole lot of everything else. I would suggest you don't need a gray tank if you always park in a campground where sewer connections are available. You can't dump gray water on the ground while boondocking.
This is a classic "while your at it" dilemma.
If your subfloor is out and the belly pan removed, installing waste water tanks is easier. My waste water tank project cost in the $1500 range for two tanks 500, freight 100, grommets, tank hangers, valves, ABS pipe and fittings 200, and a big tank cover 400. I also opted for a new SeeLevel tank water level monitor 230. And a whole lot of my labor, which is free according to my wife.