Like has been said, it depends on the model of transmission you have. In my 2005 Chevy 2500HD (same transmission as the same year C motorhome with 6.0 V8), tow/haul changes the shift points for better towing performance. It does NOT keep the transmission out of overdrive but it will drop into 3rd more readily. It does nothing on downhill grades. Newer models like the 6L80E have grade braking.