Did you buy your unit used or new? If new, the tires on one side would not be older than tires on the other side .... however depending on your location like in the southern part of the US, the tires on one side may have been exposed to the sun 24/7 so 'look' older.
As for wear, uneven wear is generally caused by an axle out of alignment. Remember, when turning, especially when backing up making a tight turn there is alot of twist on the tires/wheels and axle. An axle out of alignment is not unusual on a trailer of any kind.
Any decent alignment shop (one that does trucks and trailers along with autos) can check the axle alignment and correct it, if necessary.
2012 Fun Finder XT 276 Toy Hauler & Harley Davidson Ultra Classic with Condor wheel chock
2014 Dodge Ram 2500 AT 4X4 Crew Cab 6.4 L Hemi, 373 gears, tow package, Rambox option, Revolver X2 tonneau cover, Equal-I-zer anti-sway hitch.