I tow a 4 door Silverado behind a DP. I definitely know it's back there and can occasionally feel a little push on the back of the coach in certain situations. I later added a Polaris RZR to the bed of the Silverado. It put the truck at it's carrying capacity, so I added air bags to the truck for some additional stability. I was still 2500 pounds under my tow rating for the motor home.
With that said, you are pushing a SHORT Class A gasser past it's limits and now adding weight up high in the bed of the truck. I would be worried that any type of evasive maneuver while on the highway would send the vehicles going in two different directions, while still connected and could be disastrous. I think cautious and smooth driving would allow you to go thousands of miles without issues, but one maneuver to avoid an accident or obstruction in the road and you'll be out of control.