We lost the rear two during our 2012 trip - I never replaced them. My brother chased one down early on the trip as he and his GF was following us, then sometime later, it disappeared again without anyone seeing it go away somewhere in Utah or Colorado. The second one decided to part company a week later, probably over those northern California Stop Ahead rumble strips. Those things feel like they're gonna take the suspension out!
I figured I'd buy 2 new ones when I sold/traded the rig. Otherwise, gray wheels are a whole lot cheaper than lost wheel covers. I've thought about painting them bright silver. (I've thought about it, that's as far as it went)
The bolt-ons would be the way to go.