nitrogen filled tires

In response to OLD BISCUIT - yeah I know our atmosphere is 78% nitrogen and I have always thought as you did: what's the point? The more I read though, the more I am beginning to understand the issue. As it turns out, the issue of leaking oxygen molecules is the least of the problems with atmospheric air - although that is why Nascar uses nitrogen. The real problem is the oxygen in the tires tends to oxidize tires from the inside out. Beside the fact that nitrogen is (for all practical purposes) inert it also tends to hold and contain less water vapor than atmospheric air. At least that's the explaination I have read for it's use. Having said all that - I still agree that it's hardly worth the effort to drag a 13000 lb trailer around with low tires looking for a nitrogen fill. I thought that just maybe somebody knew of a convenient method of maintaining the original nitrogen fill. Thanks anyway for your thoughts.