This is my opinion, and others may disagree.
I think that any external wheel bearing grease system (eg, EZ Lube) on a travel trailer's axles does not make sense and is in fact not helpful. On a boat trailer, likely a different story due to frequent immersion in water. But a proper grease job on a travel trailer's bearings will last many thousands of miles and at least a couple of years. There just is no need to inject extra grease. After a couple of years or many thousands of miles (I think 24,000) its time to remove your drums, inspect the brake system, and clean and re-grease the bearings. But, adding extra grease through a zerk can cause an internal seal to fail, and to get grease all over your drums and shoes. I understand that spring loaded wheel bearing covers have a relief valve to prevent busting the seal, but since (in my opinion) adding extra grease is not needed, there is no point in adding spring loaded wheel bearing covers.
Good luck with your trailer; thinking about wheel bearing maintenance is a good thing to do!
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch