We sometimes tow with the 86 gal fresh water tank about 1/4 full in case we want water while on the road to our destination. We have never had a problem with sway and we use a Reese Straight Line Dual Cam Weight distribution hitch. It is fantastic.
http://www.reeseprod.com/content/products.aspx?lvl=2&parentid=1600&catID=1665∂=0
Even with bad cross winds we have not experienced sway when driving and have seen other trailers swaying all over the place.
We also put a water filter inline from the water source to the tank when filling at ALL fill up places. Its amazing how the filter takes out all the minerals and gunk from the water.
One person mentioned about putting some water in the black tank and towing to get it clean - we also put in some Calgon liquid and that helps the stuff slide off the sides of the black tank. Its just a water softener liquid.
Angela and Gary