That Jacking them up OFF the ground iS what I do to my little covered trailer.
SUN is the Ruination of rubber though...
You really got Time out of them and sounds like they were Kept in Good Shape...... Good for you.
There are some Motorhome, Trailer supplies that discount tires But like Steve says Make sure they are for the trailer and correct # for the weight total also...
On my little trailer, keeping Off the ground, I give tires an occasional Spin to re-locate lube for the bearings. I don't know IF spinning does anything Real Beneficial BUT I feel better doing it.
I prefer NON radials..... IMO On A Trailer (radials do Heat and Flex too much in my book)
signed....BUBBIE