You specify said "RV8s" so I'll assume not an A model? I would say you could land a RV-8 on grass, gravel, tarmac, even paved runways. A concern may be gravel or soft ground and how much clearance you have in your wheel pants (if installed) or if the pants aren't installed, are you willing to risk damage to the horizontal from rock chips. .
RV-8 "can" land on gravel, dirt, grass, asphalt, concrete. Just can't tolerate excessively soft surfaces, rocks/gravel larger than ~1", uneven surfaces. Paint dings and chips are the price you pay for gravel and small rock operations.
I would operate off smooth maintained grass or tarmac. But I’ve seen enough landing gear issues with the -8’s (main landing gear bolts coming loose, tailwheel stingers wearing and requiring oversized fasteners, tailwheel weldments coming loose) that I’d have my reservations operating off anything unprepared.