It really depends on a lot of factors: the weight distribution of your trailer, the sizes of the tanks, where you've loaded your other gear, etc. I would probably try filling the freshwater tank full (leaving the gray/black tanks empty). One other important thing to remember is to load any gear that you keep in the trailer in the front of the trailer. Too much weight in the rear is likely to cause sway.
If you really want to get technical (or dorky, whichever you prefer

), you could take your truck and trailer to a weigh station and weigh each axle to determine the exact weight distribution. I haven't yet done that with mine, but I plan on it.
Did I mention that I like the Shadow Cruisers? Hopefully we'll meet up in the near future and you can give me a tour of your rig!