silversand,
Very well said because even a mildly rusted steel screw is hugely diminished in it's holding strength. The rust always occurs on the threads and the OD of the steel screw solid shaft area (minor diameter) which as about anyone knows. then the OD below the thread depth on a rod, shaft, or screw below the thread bottoms becomes a much smaller diameter of actual metal than the listed screw diameter specified by size. Rust only .1 mm to .2 mm deep (.004" to .008" deep) compromises the screw shank (minor diameter) shaft strength by 25% to 40% and the actual threads strength by 50% to 90%. The OD of a diameter give the most strength as it's the circunference and not only the longest distance around the diameter but circles the diameter with it's 2 separated surfaces held apart by the inner metal. That's exactly what makes roof trusses so strong and supportive and airplane wings so strong etc.
What good is a screw with the threads rusted thru and holding NOTHING or the shank strength nearly cut in half? As said, there are so many strength varieties in SS screws and even the weakess SS screw will hold a lot more than a marginally rusted screw which also rots the wood around it very fast.
Meanwhile, a SS screw constantly maintains it's strength and thread holding ability for thousands of years at min. The material the threads are in will deteriorate/be compromised first, not the SS screw. Can't see anyone keeping their RV that long but who knows since longer living is a work in progress?
