7800lbs will equate to around 900lbs on the receiver. That will drop the rear and raise the front of the truck. Depending on how much the front raises will determine if you need a WD. You don't want too much weight lifted off the front tires. FWIW I towed a similar size TT (7300lbs) that had a 900-950lb TW. The rear sagged about 1-1/2" or more. The front only rose up about 3/8-1/2". Not really an issue for me. But I have the big heavy CTD sitting over the front tires. If it were me I'd use a WD to return the front to its original height. Really only takes a couple minutes to hookup.