the last set of tires I replaced actually had good tread but were dry rotted and cracked on the edges from age.
On a trailer I don't think its worth risking the load which is probably a mopar and risking someone's safety over a set of dry rotted tires .....when replacements are only a few hundred dollars every 5 or 6 years.
I actually trust older tires more on my dart where I have better control over a blow out .....when you have a blow out on a car, you can feel it and react fast .......when there is a blowout on a trailer, sometimes by the time you realize, the reaction time is much slower and the consequence could be a lot more damage because every second counts on the highway. AT least that is my line of thinking.