Although I'm not a believer in the five year replacement, you can't compare farm and construction equipment tires with on the road tires.  They are subjected to no where near the speed or heat.  We have a fuel truck at an airport where I work with a group that has tires that are very old on it.  I have no idea the age but they barely hold air for more than a few weeks and have huge cracks along the sides.  The truck is moved at about 3 mph on a concrete pad about 500 feet long and that's it.  We'll use the tires until they fall completely apart as there is no danger in using them.  But I sure as heck wouldn't run them down the freeway on my motorhome!