There is no way to render the entire warranty void. If you do something that damages a part on the car, the repair of that part will not be covered under the warranty.
There are three sources for towing receivers for the Sorento, the stealer, Curt Mfg., or the Cequent brands (Drawtite, Reese, HiddenHitch). They all work fine. As long as you do not tow more weight than your owner's manual says is the maximum, you'll be fine. If you have the V6 and tow the max, you need trailer brakes.