Fresh water AND grey water combined?

There are far better solutions. Since the OP is talking about a brand new and never used Vehicle, then I assume he's not purchased it yet. Simply look for a trailer with a very large FW capacity. I have a 2011 bottom of the line Starcraft 235fb that has 90 gallon FW capacity. I bought it because I wanted the ability to have a lot of FW. There are many trailer models to choose from and some have very large FW tanks. If you want even more water, there are bladders for your tow vehicle, Gerry cans, 8 gallon portable containers, etc. Trying to modify a gray tank just doesn't seem smart. How will it impact the total weight of your TT? Usually they figure you start out with full FW tanks and as you use it, the black and gray tanks fill up. Traveling with full FW and GW tanks may leave you little spare weight for everything else (food, gear, etc) that you will want to take.