I replaced a tri axle steel trailer with an aluminum tandem axle trailer. The new trailer has a tendency to sway at highway speeds. Do I have too much or too little tongue weight?

You need more weight on your tongue,I think it's 10% of your weight should be on the tongue.

It could be either, but probably too little.

Set your truck up so it is sitting fairly level, not all sagged out in the back, and add more tongue weight. Most of the weight in a boat is in the rear anyways so the axles on the trailer need to be pretty far back. Check truck and trailer tire air pressure also. Good luck.

Make sure the trailer is level as well.

Thanks
Ray