I read that tire polish is bad, covers and exercise good. The advantage of Nitrogen is that it is dry while air can have quite a bit of moisture which reduces tire life. The 2006 Michelyn tires that came with our MH were cracked like the OP's in 2012 when I replaced them.
I remember talking to a Goodyear dealer about tires that didn't last. I told him I had an old van parked for 12 years and still holding air (Michelyn) while his tire had lost its air after three years. His answer was that they don't make them like they used to. Probably weight, sun, oxygen, usage and manufacturing goofs all have thir effects.
