I'm quoting an answer that I got from someone on this forum.
"Uhaul bearings are sealed. You do not grease them... the grease in them is never changed. You don't do anything with them until they wear out, and you can tell that they are worn on inspection because they have too much free play, which is a check that should be done with any wheel bearing, sealed or not. When they are replaced, they are usually part of an assembly which bolts in. Bearings work very well and routinely last the entire life of a car which travels ten times as much as a trailer. "
When I recently had my trailer in for new tires
, my mechanic said this same thing. He tested the 'wiggle' in my wheels and said that they were tight and not needing any work.