Ron,
The touching problem can arise as, or if, owners reduce tire pressures to improve the ride and/or handling. Thus it would be highly variable as to when it will happen. It will depend upon the motorhome's weight on the dual tires, versus the pressure being used, versus the actual spacing between the tires (which is controlled by rim geometry and tire body width).
I went conservative because of our sometimes hot weather travel combined with off-pavement travel. This requires 80 lbs. pressure in the rear tires in addition to slightly narrower tires.