[wingnut60]

Stanley,
Do you run TP sensors? If so, they would be very useful if you want to go to the trouble to keep track of how fast the pressure rises from cold over time at lower psi. I believe you will find that the lower you run the psi at any given load, the faster/higher the psi will rise, along with temp, when towing. I do not think you can do much better than run the sidewall max psi (which will be either 120 or 125 for most 17.5s).
We can get caught up too much with specs/opinions/tables/charts--which may be helpful for lower load range tires--but for the 17.5s, just run at the sidewall max and enjoy the trip.
Joe