A gray tank it needs to be near the black tank for ease of dumping, it is easiest if both discharge lines merge after the valves. Consider where the existing gray lines are so you don't have to make major changes in them. When you mount this remember water weighs a little over 8 lbs a gallon.
If your fresh water tank is good changing it is going to be a trade off. A plastic tank will require a pump, and they are the subject of many complaints because of noise. I would try it for awhile, if it doesn't work, change it later.