You should always know the exact weight and ratings for all your equipment. If not - get your rig weighed. Overloaded suspensions can contribute heavily to sway. Try different configurations. Weight it empty, then fill the water tanks and weigh it again. Then completely pack it and weigh it again. Know how each configuration affects your tongue weight. You may be surprised just how much each of these configurations weigh. It's probably more than you think.
The only true way to determine your true tongue weight is to take the trailer (loaded as you would for camping) to a scale and have it weighed. While you can use a scale at a rock pit or trash dump ( or even a bathroom scale for a single tongue weight), these scales usually have only one platform. It is actually best to use a multi platform scale in order to see what your weight is on each axle (1 Steering axle on tow vehicle, 2. rear axle on tow vehicle, 3. trailer axle) as shown in this diagram.
After you get these three weights, leave the trailer on the scale, but disconnect from the tow vehicle (remember to always chock the wheels) and pull it off the scale, leaving your trailer axle on scale #3 and your tongue wheel on scale #2. The weight on scale #2 will be your exact tongue weight.