If your trailer has ordinary axles with ordinary wheel bearings, then repacking the wheel bearings is a messy but relatively easy job.
Jack up the axle, remove the tire/wheel. Remove the grease cap on the end of the axle. Remove any excess grease on the end of the axle, including any grease in the grease cap or in the cracks and crevices around the end of the axle. You will see a nut with a cotter key through the nut. Remove that cotter key and then remove the nut and washer. Then wiggle the hub, and the outer bearing (and maybe more washers and seals) will come out. Then remove the hub and you can get to the bearing on the other side of the hub. (Removing the hub may require disconnecting the electric wires that go to the brake magnets.)
Clean the grease off the bearings with a clean rag. Then pack the bearings with new wheel bearing grease. You can hand-pack the bearings if you know how. You must get all the old grease out of the bearings and replace all
the old grease with new wheel bearing grease.
But using a simple wheel bearing packer tool is much easier to do a good job than trying to do it by hand. I know how to do it right by hand, but I prefer to use my wheel bearing packer tool. Here's a link to one:
Original E-Z Squeeze Bearing Packer Tool | A to Z Tool
After the bearings are repacked, then reinstall everything in reverse order - including the nut on the end of the axle but not the cotter key or grease cup yet. If there are any grease seals in that assembly, then discard the old grease seals and replace with new ones. When it's all assembled, turn the hub while finger-tightening the nut on the end of the axle. You want the hub to turn freely, but with no "play" in the bearings. You want it good and snug, but don't use a wrench and get it too tight. When you have it right, then install the cotter key in the nut, replace the grease cap on the end of the axle, and install the wheel and tire.
Repeat for each hub on the trailer. After the first one you will know how many and what kind of grease seals you might need to do the other hubs.
Here's one YouTube video that shows a simple axle. His bearing packer is not exactly like mine, but it works. And his trailer has a "retainer" instead of a cotter key to prevent the nut from turning, but it does the same thing.