towing problem

i have alot of seat vibration while towing my bass boat. i took it to the dealer and had service manager ride with me. at 70 mph it shakes the rear end so bad that it will slosh your drink in the cup holder. we took it back to the dealership and hooked my boat which is a brand new boat to another 2015 tahoe and had the same result!Dealer said they have had vibration issues in the escalades and will report problem to gm. I have towed other boats with mine to eliminate the possibility of it being the boat trailer with the same result. when i tow with my sons 2014 i have no issues. Im hoping they let me out of this vehicle as i tow several days a week and its unbearable. anyone else have this problem?

Check the tongue position of the trailers. The new Tahoes typically need to have more drop on the trailer ball than older models (the hitch position has changed). When hooked up and loaded, the trailer tongue needs to be level or pointed downward. A slight uphill cant to the tongue will cause the issues you describe.