On any SuperDuty with the 5R100 (TorqueShift) tranny, using tow/haul will not disable overdrive - can't be done. The only thing tow/haul does is up the line pressure to reduce slipping and improve cooling, and give you the feature of 'engine braking'. Fifth gear in the TouqueShift is overdrive and no way to lock it out unless you manually do it via the gear shift, but then you would also lock out 4th which is the final gear (1:1).
You need to read the owners manual - it specifically states that you 'should' use tow/haul anytime you have anything in tow, or are carrying any significant weight in the bed.
Yes, you can run without putting it in tow/haul, but if at some point you have driveablilty issues and you tell the dealer that 'it happens when you tow', their first question will (and I guarantee it) be 'are you using tow/haul?'
It's on the 5R110's for a reason ......