Tow/haul mode doesn't disable 6th, it just holds the gears a little longer and downshifts a little quicker.
It also enables engine brakeing.
Yes - you should use it.
Tow/Haul delays upshifts to reduce frequency of transmission shifting.
Tow/Haul also provides engine braking in all forward gears when the
transmission is in the D (Overdrive) position; this engine braking will
slow the vehicle and assist the driver in controlling the vehicle when
descending a grade. Depending on driving conditions and load
conditions, the transmission may downshift, slow the vehicle and control
the vehicle speed when descending a hill, without the accelerator pedal
being pressed. The amount of downshift braking provided will vary based
upon the amount the brake pedal is depressed.