This site should answer your question fully:
IMHO if you wish to be presenting your CT in as close to original as possible then I would go with bias ply tires. And if I were doing alot of back road/off road trailering then I would chose bias ply (if for nothing else but sidewall protection).
If one simply is trailering on paved highway or on gravel roads and wanted to have the trailer go down the road with the least tracking problems and with with better fuel economy, then I would go radial. On our light weight
trailers weigh carrying capacity is of less concern, no need for class D tires.
Hope this helps,
The previous owner on my Uhaul CT13 used Wanli S-1015 155/80R13 tires. The tires are basically new. They are radial tires however. Is that ok? I was under the impression that the CT13 should have bias ply tires.