I would like to add to this...
I agree entirely that it is a crap-shoot. Although there are also a few human and natural causes that will decrease the life of your tires. You say that your tractor tires are still fine but your brother in laws honda tires crapped out at 4 years-
Being a Honda nut and avid carshow enthusiast, I know (and I think we all do) that people who love their cars often treat their wheels with tire shine products, or some other form of tire spray. These products, with the exception of some higher-priced brands of sprays, almost always decrease the life of a tire, as it will draw out the natural oils in the tire, and the tire will dry up/crack. Granted, I don't know your brother in law, but if he loves his car, and sprays his tires, this will decrease the life of the tires. I've seen it many times. There are also tire cleaners/acids that will draw the oils out of the rubber. 'Brite-white' or however it is spelled is used for exactly what it sounds like; to make your white walls bright! It also cleans the whole tire and I am guilty of using it on all my tires because I just like clean black rubber, that's how I am. I advise not to use this or any tire cleaning product on your tires, especially on your rv, where you will most likely run the tires for years and years.
My opinion: Your best bet for keeping your tires from cracking is to leave them alone, and store them off the ground or inside during the off-months. If you really want, you can buy products that claim to prolong the life of your tires (from cracking), but I don't have any definitive proof that these work.
As far as your tractor, well it probably sits comfortably undercover or the rubber is 2 inches thick and that would be my best guess as to why the tires are still a-ok. Either way, be glad you don't have to replace those tires every 4 years...I am sure that price tag would have a big 'ouch' factor.