Tires

I assume you are refering to the speed rating on passenger tires. If this is right, a speed rating of T means the tire is rated to 118 MPH and a rating of H means a rating up to 130 MPH.
Speed rating is not a good way to compare the wear and handling capabilities of one tire over another, especially among different brands.

We now have a tire designed especially for RV's (G670 RV). Whereas the G159 was a truck tire that was used on RV's the G670 RV was designed to be used on these vehicles. When it comes time to replace the tires, I suggest you have the aligment checked and try the G670 RV tires.

On any vehicle, if worn tires are replaced with new tire, there are often complaints of squirm or an unstable feel. Since the new tires have much deeper tread depths than worn tires, this can be a case of just getting comfortable with the way new tires feel. Also, 8R19.5 tires are what we refer to as "standard aspect" tires as opposed to "low profile" tires. Depending on vehicle, you might be able to replace the 8R19.5 tires with 225/70R19.5 or 245/70R19.5 tires that will make it more stable by lowering the ride height. Check with your vehcile dealer or manufacturer to make sure these tires will fit your RV. PS. The smaller diameter tires will effect your speedometer, odometer and gearing.

Tire tread depths are typically listed in terms of 32nds of an inch. For example, a new steer axle tire for a line haul truck might would be about 18/32's of tread depth when the tire is new. "Wear of 6/32" should mean that 6/32nds of an inch of tread has been worn away.

Stone drilling is, unfortunately, common for tires that travel "off highway". Goodyear and the other major tire manufacturers make tires specificly for off highway applications. If you are using highway tires in an off highway situation, that could be a large part of your problem. Check with you tire supplier to see if the tires you are using are the one's best suited for the roads you travel.