I had to put 6 12x22.5 tires on my old RV and switched the steer axle to the drag axle as those from the front were only 3 years old. Not a pleasant day as those things are almost $500 a pop. And the bad part was there was lots of tread left on the 10 year old tires. I just did not trust the old tires with 120lbs of pressure.
I am not sold on nitrogen as the atmosphere is 80% so we are only gaining about 20%.