The question of Nitrogen being used to inflate a tire has been covered in a few of my blog posts and on some other posts on the internet. It is often mentioned that the Nitrogen used to inflate tires is "dry" and so that is considered a positive but I don't recall anyone getting too specific on why "dry" is better than "wet". Before we get into that part of the discussion I want to be sure that everyone understands that any "gas" you buy in a high-pressure tank aka cylinder, will be "dry" as the process of separating the gas and compressing it removes the moisture. So if you were to buy a cylinder of CO2 or Argon or Helium etc the gas would be "dry.
I did a post in May 2012 as one of my early posts on How to get dry air for your tires by making your own air "dryer" for less than $20 for an almost endless supply any time you need it.
There are some companies that sell small cylinders of various gasses but you would also need to have a pressure regulator and a source of high pressure (1,000 to 2,000 psi) gas to refill your tank.
But back to the original question of why do we want to keep moisture out of our tires and why would getting "dry" or at least "dryer" air for our RV tires be advisable. By "Dryer" I am referring to the wet air you can get out of the "free" air at some gas stations where they might not maintain their air compressor or air dryer.
In the past when RV trailers came on Bias or tube-type tires we didn't worry too much about dry air primarily because the tube did a pretty good job of keeping high-pressure moisture out of the tire carcass plus those bias tires did not have steel belts and were probably just Nylon or Rayon cord.
The modern radials we use today are almost all "Steel Belted Radials". So the question is what happens when you "mix" steel and water? The steel can rust over time. Now in most tires on your car or TV you may drive it almost every day. The driving generates heat and this heat is highest at the belt edge which would be the most susceptible to moisture. This heat tends to drive the moisture out of the tire rubber. However, if you let the tire sit for days or weeks moisture in the air can migrate into the tire structure. Moist inflation air can be driven into the tire structure and over time this moisture can attack the ends of the steel belts and form iron oxide. Rubber doesn't stick very well to the rusty steel so tears can initiate at the molecular level. Once cracks or tears are initiated in a tire they can only grow and if allowed to grow long enough or big enough you can end up with a separation in the tire structure.
Keeping moisture out of the rubber structure is why we also recommend you not park with your tires on wet sand or dirt and the moisture can migrate into the structure if exposed to water for weeks at a time.
Please don't jump to conclusions and say "Ya but I drive in the rain" or "Occasionally it rains and my tires get wet". I am not talking about a few hours of exposure or even a few days. If you drive and heat up the tires it will drive the excess moisture out of the tire. It is weeks or even months of parking in a wet situation that we want to avoid.
Not inflating your tires with wet air (if you get water drops spitting out of the air hose it is way too wet to use except in an emergency) is what we would like to do. If you remove your valve core and what looks like fog or steam or water droplets spits out you have too much water in your tires and this is under high pressure all the time so this can affect the life of your tires.
Many after market tire sealants are water based so using that stuff can hurt your tires.
So that is why inflating your tires with dry or at least drier air is a good practice.