Re: metal vavle stems
First, how about making what you post easier to read by not using all caps? In case you were not aware of it, most forums consider all caps to be very discourteous.
Changing to a tire that has a higher ply rating will not raise your RV by 5" and probably not enough to even detect it. If the tires have the same size numbers as the ones that you have now, the tire diameter will remain the same too. If the dealer does intend to put on a tire with a larger diameter than you have now, and if it is enough to increase the clearance by 5" that would mean that the tire diameter will be 10" greater and I do not see how it can possibly fit into the wheel wells.
Section Width
Following the letter(s) that identify the type of vehicle and/or type of service for which the tire was designed, the three-digit numeric portion identifies the tire's "Section Width" (cross section) in millimeters.
P225/50R16 91S
The 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tire's section width. Because many people think of measurements in inches, the 225mm can be converted to inches by dividing the section width in millimeters by 25.4 (the number of millimeters per inch).
225mm / 25.4 = 8.86"
Sidewall Aspect Ratio
Typically following the three digits identifying the tire's Section Width in millimeters is a two-digit number that identifies the tire's profile or aspect ratio.
P225/50R16 91S
The 50 indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. We know that this tire size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 / 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tire size results in a tire section height of 4.43". If this tire were a P225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall.
In other words, if you change tires from a middle number of say 50 as in the example, to one with an 80, that will mean that the tire has a height that is 7.09" or 2.66" taller. To achieve an increase of 5" would be very unlikely unless the tire is also wider than the present tire. The problem with that is that a wider tire will probably not work for your dual rear tires due to tire spacing. As the tires get wider, the space between the two dual wheels remains the same so the larger tires may very likely rub together as you travel, very quickly destroying those tires.
Be very careful when changing tire sizes used on dual wheels.
On the metal valve stems, on an RV I would only use metal stems. They are much safer and especially so if you are using valve extenders. And for extenders, use the ones with the braided steel outer layer.
