Fair warning - this post is loooong and and even more boring than usual unless you're one of a small handful trying to decide if they want to tackle inspecting and packing their own wheel bearings. Keep in mind that this is based on a 2010 Scamp 19. Others will be similar but may differ in the details.
Assuming the bearing cone and outer cup look good and you've carefully repacked the bearing, place the rear (larger) bearing into the hub and install a new grease seal. I use a soft block of wood and a hammer to press the grease seal in place without deforming it.
Al-Ko manual (page 23) has specific instructions although most experienced mechanics do it by feel. In spite of the manual, install the cotter pin so that it does NOT go over the end of the spindle.
I got all the parts from Etrailer.com. The #84 spindle is very common and parts are readily available.
Just as when the trailer was new, check the hubs for excessive heat at your first stop after servicing the bearings.