I think Mackerr is talking about finish-grade plywood rather than underlayment. In lieu of using vinyl flooring.
5 mm is roughly 0.2 inches and equal to the finished thickness of 1/4' plywood. Such plywood is usually soft wood (e.g. pine) and not well suited for use as flooring. The finish scratches easily and the surface is soft enough to gouge. Unless that plywood is being sold specifically as flooring, I would not even consider it as the finish flooring. And even if it is, I would be skeptical about using it in an RV. If its a hardwood such as oak or maple, the wood itself will hold up ok, but the finish (varnish?) is still going to scratch readily.
Technically, Luan is considered a hardwood but when used in plywoods and veneers it is very, very thin and quite soft. Not very rugged at all.
Rv floors are inevitably exposed to sand and gravel under our feet and wide swings in temperature and humidity. The flooring should be a material that is very stable and resists scratching. I would stick with a premium grade vinyl plank (e.g. Allure Ultra) or premium sheet vinyl. The better grades are often hard to distinguish from real plank or stone. Since an RV floor is small (in square feet), a premium grade floor covering doesn't add much to the cost.
If a wood floor is attractive to you, get "engineered wood" flooring in square tiles or planks.
