everybody speaks of grease for bearings - but why not oil ?
Please have a look at any bearing data sheet, the max speed acceptable with oil is grease x 3 ...!
I did it on my own trailer 25 years ago, I built it 100%, and I used oil, nothing else.
This trailer is close to dead because of internal rust - even after having heat-tinned all when it was new, but inside ... - but the bearings are like new - I never added oil.
I modified several other trailers in the same way :
I added a thin sleeve made of brass (lathe work) to protect the rubber against rust, then I epoxy-glued it on the shaft.
2 holes into the hub (oil-in and oil-out), plus tight caps glued with Sikaflex.
Oil is Dexron 3, but any oil is ok.
Check twice the year for water, if any, replace the oil.