I love having rollers on the back of my trailer. If I had it to do over again....
One of the simplest things I've ever seen is to have a 1 1/2" steel pipe going across the back of the trailer with a 2" PVC sleeve on it. With an inch or two clearance below the bar, the PVC sleeve will spin even if the bar gets warped down a bit. No bearings, nothing fancy.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse