Irwin, my sarcasm meter must be busted, so I'm gonna take your question as serious, as there are varying degrees of "clean" so here goes.
Exterior: paint/decals should not be very faded or worn; just a few to no scratches or big dings or damage; tires in good shape;
no cracks or breaks in windows; seams should be flush, no gaps; door should open easily and shut with a good seal; steps should work ( manual pulls down and folds; slides work properly with good seals
back up easily, electric steps the same.); ladder to roof should be in good shape. Roof: roof coating should have no obvious tears; good seals around the vents antennae and AC/heater covers; good seal around edge of roof.
Interior: Ceiling- no sign of leaks at all; floor- no soft spots; tile, wood or carpet show only normal wear and not dirty; furniture; in good shape again showing only normal wear and not dirty or overly stained; appliances in kitchen- in working order; not dirty, fridge doesn't smell like socks; all cabinets and drawers work properly; faucets work, no major drips. sink not scratched to hell or deep stained and drains well; bathroom- everything in good shape, sink/faucets work properly, drains well; tub/shower combo not dirty or gunked up, shower works, tub drains well, enclosure not cracked or broken, shitter in good shape and works properly, no turd residue/skid marks left in bowl. Bedroom- floor not dirty, normal wear; cabinets, wardrobe, drawers all complete, work properly,not cracked or peeling, mirrors no cracks. don't care about the mattress, I have my own. Cockpit up front- captains chairs in good shape; all gauges, levelers, etc work properly
Mechanicals/drive train. Everything needs to work. It is a used vehicle, but it should be in good shape.
So, I semi jokingly said all that (I know, TL;DNR) to say I'm looking for an RV that's used, not perfect obviously, but in good shape. Price I'm willing to pay depends on how good of shape the rig is.
Cheers