Personally, I do my own bearing service and my current TT is on it's 11th camping season.. I do them every other year..
I don't have the zerks to add grease, but the axle is setup to have them.. There is a huge area where you have to pump grease into so it fills the void and I've never felt that was necessary... Just the bearings greased up as I've done to vehicles since I was a kid and my Dad showed me how... :)
Anyway, I think the only good the zerks are for is for boat trailers that get submerged in water and all that grease in the void probably helps keep water out of the actual bearings...
But, I see a lot more boat trailers on the side of the road with a missing wheel than I do TT's, so there you go.. ;)
Good luck!
Mitch