If you add the TT to your truck and the back end drops, you are adding weight to the rear axle and taking weight off the front axle. This reduces your steering and braking.
When you add a WD hitch and the TT and truck should sit level again.
This is what the WD hitch is supposed to do.
It is easy enough to check. Weigh your TV on a CAT scale, then hook up the TT and weigh the combo. From the weight change you can tell the tongue weight and changes in the axle loads.
As already pointed out, without a WD hitch, you my be exceeding the tongue weight of your TV with the TT attached.