Do I measure the diameter of the hub? And that's the size dust cap I search for?
Take off the one that you have and go to an auto parts store with it. I would buy a pair of the cheap ones without the rubber plug in the end because I will never use the troublesome grease fitting anyway.
curious, why are the grease fittings troublesome and why won't you use them? my first rv and new here.
You are to regularly inspect the brakes and bearings. To do that, you need to remove the hub to inspect the brakes and then the grease seal to remove, clean, inspect and pack the bearings with grease. If you want to pump grease in there you will need to keep pumping grease in till fresh grease comes out which is a mess and a needless waste and makes future service a chore. Often the grease pumpers push grease out thru the rear seal, destroying the brakes. That grease fitting is nothing more than a sales gimmick for the lazy or unknowing.
Howdy Wazkaren, When you do find a replacement bearing cap and get ready to install it, jack up that tire and check to ensure the wheel bearings are adjusted properly. Just grasp the tire 180 degrees apart and try to wiggle it. Should be mimimal "wiggle to it. There are some YouTube videos you can watch to help with the inspection process.
Sometimes, if the bearings are wearing or going out, it will cause the hub to move around in a manner that causes the dust cap to come off.