[Zeke]

If you do a study on the tow/haul feature you might find that it's mainly modified shift map. I don't think it disables OD and certainly does not disable lock up. If you feel a bit of a drag due to a modest incline, drop the gear manually. Just like a Porsche, you want to keep the RPM's up. In your case, maybe 1800 rather than 1500 with a load due to incline or headwind.

Those figures are only a guesstimate.