With our last 5er, the OE ST tires would lose around 2-3 psi per month. After all 4 suffered tread separations, they were replaced with LT tires, in our case XPS RIB's. I continued to monitor pressure as I had done previously and found that they lost 1 psi in a 12 month period. On top of that, the rig towed better and the truck actually got a little better towing fuel economy.
ST tires carry a warranty of 2-3 years of which few actually make it that far. The LT tire we bought had a written 6 year warranty from Discount Tire. There are many reports of tires lasting over 7 years without any problems.
So, you can buy cheap ST's every 2 years or so plus the cost of RV repair following a tire failure or you can pay a little more and go happily down the road for 5-6 years of trouble free RVing. The only requirement is that the tires you purchase must equal or exceed the axle rating. Your choice.
Keith