Looking for the best hole filler on the inside of our RV

DrewE wrote:
For screw holes in the luan, I'd think plain old spackling compound would work just fine. There's not much flexing going on in the 1/8" hole itself. This has the advantage of responding reasonably well to being sanded smooth once it's set.

If you do want something else, some sort of a paintable caulking compound would be fine. This is one of the very very few places where silicone would be appropriate for use in an RV, even, in my estimation.