I've put several trailers in service. Before any of them were put on the road I did a bearing/brake R&R on each new or used unit. Its suprising what one finds from the factory. I still do all my rv and non rv trailers the same.
I've never carried extra bearing even when I was on the road. No need to if you've did proper maintenance. If you haven't then it may be a good idea to carry a spare.
On the rv trailer I open the hubs every spring and make a visual on the bearings....them make a decission if they need attention. Same with my other trailers if they see lots of miles.
Now my 18' car hauler doesn't see that many miles annually. I did my brake/bearing R&R when it was new and open the hubs maybe once every 2-3 years for a looky. Its a '11 model and has recieved one repack.
The boat trailer ??? I've found replacing the BB with new water tight cap eliminates most water intrusion issues.