Probably, some mice are darned small.
But, how would they scale the vertical side first? Toeholds are tough to come by...
Be much more concerned about the power cord, tires, tongue jack and stabilizers as paths to the RV. Then they have full run of the framework, suspension parts, wiring, plumbing and propane lines.
Where these lines enter and exit the cabin, is where we have to close any gaps.
Not just mice. Insects just love to settle into quiet places out of the full sun or rain...
And don't get me started on wire-munching chipmunks!