Most have a removable center cap on that chrome hub. All of mine have. If you look close, there is a small spot that a flat head screwdriver can squeeze in and pop the chrome cover. Then, the rubber plug comes out. Not sure which wheels you have, but if they have EZ lube hubs, then your hub cover SHOULD have a removable snap in cap. It just doesn't always come out easy. I have broke them before.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.