In a related question, Tools and supplies for roadside trailer bearing repair, @Pᴀᴜʟsᴛᴇʀ2 said:
Something to think about is to worry about preventive maintenance more than worrying about having a breakdown on the road. If I were you, I'd put zerk caps on the hubs so you can push new grease into the hubs very easily.
The number one killer of the bearings on these is because of water ingress. If you check the seals and keep it greased regularly, it becomes a non-issue and breakdowns due to the hubs will become non-existent. One of the issues is, if you suffer a hub breakdown, you'll do more damage before you notice it than what you can repair on the side of the road (most likely).
Wheel: 5.70-8 4-bolt
What is involved in installing zerk caps on boat trailer bearings, and what tools and supplies are required?
I'm going in green; I don't have any experience with bearings/grease/trailer mechanics.