And what maint is required? This is off my new trailer, just getting going up here, wanted to pry the cap off but it didnt look like it wanted to. Someone on here will know this.

Is there a grease fitting in the center of the cap? If so bearing buddy. Usually there will be a sticker on the trailer near the front/ Check inside the frame rails - they hide em

that one ie called your phucked style...need to remove the wheel to get at the grease zurt....then either get center caps that have holes in them or tale a hole saw to yours..

Usually those center caps just pop off with a screwdriver to grease the spindle, probably an e-z lube hub behind there

Looks like a UFP Vault system Do nothing for 5 years

No grease fitting in the centre of the cap, its hard plastic......def doesnt want to just pop off, I guess I'll pop the wheel off and have a look, drill a hole or pop the centre out completely. Thanks

pop that plastic cover or drill it out and add bearing buddies, if not the wheel will have to come off every time

That pic appears to be a solid center cap. Those are not made for servicing. If the spindle is drilled with a grease Zerk then a serviceable style cap for the hub and wheel can be used. If no zerk then "bearing buddy" style caps for the hubs are the other option.
These wheel center caps could be modified or changed out for serviceable types of center caps.

I'm in the same boat (so to speak) as you. It looks like my hubs for the aluminum wheels goes on from the inside of the wheel like yours. I will soon investigate this as I want to go through the brakes every year. I don't like the idea of just drilling a hole in the end of the existing cap as it would become a water trap and possibly not drain. I'll probably just remove the caps, add bearing buddies if they are not already there and paint up the hub so it looks presentable, probably with some sort of silver rust resistant paint. Let us know what you find when you remove your wheel(s).