Bad weather will surely have adverse effect on milage.
I normally tow in D (4 sp) and let the tow/haul mode do the thinking. It will adjust the shift points to suit road conditions. Only on some rare occasions when the transmission hunts up & down in search of the right gear, I then manually down shift to a lower gear until the road level out again.
I am not familiar with the newer 6sp transmission whether you can lock out automatic entirely. But leave it in Drive with tow/haul mode should be fine most of the time.
