The tires DOT are 2 at 5215 and 2 at 0216.
That means that 2 tires were made the 52nd week of 2015 and the other two the 2nd week of 2016. So they are only 4 weeks apart in manufacturer date and with a 2016 trailer that isn't significant. What is significant is that the tires are now more than 5 years old and since you know nothing of their history, should probably be replaced with new before one or more experiences a blowout.
I tend to agree with Ken about where to get the tires, but they may be willing to replace them with new but not to lower the price enough for you to do so. A blown tire can do thousands of dollars of damage to an RV if it happens at highway speeds so tires are very important.