I've got a 2015 f350 towing a 31' fiver at app. 14k and have no problem at all with drifting. Also have no problem with oncoming or passing vehicles.

 

OP,

I think your biggest problem is your pin weight is far too light. I have 1100 more pounds of pin weight than you are running, and my rig pulls pretty solid and steady, even in crosswinds. Sounds like you're on the right track by adding some water to your tank. If the tank sits in front of the trailer axles, you may not have to add more than a 1/4 to a 1/3 tank to make the difference. I would run trailer tires at max psi, and front and rear truck tires at max psi.

Then I would get yourself over a CAT scale asap. Eliminate the speculation and scale that rig. Are you running E rated tires? Over or under on truck/payload/axle ratings?

Then, after scaling your rig, start adding water to the tank. Heck, may just try running half a tank and be done with it.

 

Thanks for all the responses! I believe the pin weight is the culprit. I did a short trip this past weekend with a full water tank and the truck was solidly steady in the same situation that produced the drift/sway. I winterized at the park, and the drift was back with an empty tank. I checked the water tank mount, and found it is securely mounted and not likely to ever fail.