Originally Posted by trwmodela
The Ford E450 tow mode is explained in the Ford Owners manual.
Basically, you use it on grades or hills but could be used anytime you are towing. What I found interesting it provides some Engine Braking too! So, I highly recommend you use it on all hills. You don't want to ride the brakes all the way down a hill.
I second Tim on the value of Tow/Haul for engine braking. It works great. Just select Tow/Haul and if the speed increases as you go down a hill, it will downshift to help hold your speed. If you use your brakes to slow down more, it will continue to downshift to help.
One thing to watch for: if you are coming to a stop and don't need the engine braking anymore, switch it back to normal. If you don't, it will continue to downshift as you slow down and the low gear ratios of second and first will stand you on your head!