Quick question: just purchased my first duramax (2015 Silverado 2500 Ltz z71). Love this thing! Had a Ford F-350 and will never own another ford. Question is when hooking up my travel trailer (around 7000 pounds dry weight and 650 pounds tung weight the rear end drops (squats) about 1.5"s even with a weight distribution system. Is this normal? Thank you!