I am looking to grease my hubs on a new Myco and it has the Air-Tight Hubs, do I remove just the rubber to get at the grease fitting? I couldn't find anything online.

Don't know what you have, but here is the install info on 4 different hubs. Hope it helps you out. The answer should be in one of those or the Assembly Instructions Table of Contents.
Would like to know how you like them and how well they work.
https://www.airtighthubs.com/pages/installation

Yes. The "bellow" comes out just like the rubber plug on a standard hub.

Thank you, I'll try on Thursday. So far, they seem to work well.

Ttt

My opinion is they are worth every .01, I have owned all kinds of boats and trailers and with the airtight you know if they leak air out they may also be leaking water in. I can tell you I've had my fair share of tires, rims, or bearing issues and nothing ruins a trip more than having issues and sitting on the side of the road.
Air tight wont solve all your issues but you will know if you have water in your bearings or not and if you do not then if you keep up on greasing the bearings with good grease every 10-15,000 miles or once a year your probably going to be in good shape.

You shouldn't need to add grease to them if they are showing full right? I've never added grease to mine bacause they always stick out the specified 3/4 inch. Should I be adding additional grease?

There full of air yes not full of grease.
Grease breaks down every 1-2 years or so. Ever leave a tube of grease sit for a year or two? The chemicals grease will start to break down and turn back to oil.
I would say at least once per year you should still at the very least pump some fresh grease into the axle, The issue is most axles only dump grease in behind the inner bearing so greasing the outer bearing is still more of a manual thing to do.
I prefer to do my trailer bearing maintenance in a garage not on the side of the road though.

I finally got around to this job, I took the boots off and cleaned any old grease out of the way and I was surprised at how much grease each one took before I saw anything move.

The biggest problem was putting the boots back on, all but one seemed to hold the air, so I reinstalled it and I'm hoping it stays inflated.

i just keep a small hand pump in the tool box on the trailer and air them up before i back in the water