My suggestion is to call Cookshack and ask them. I was told by someone there that many folks who own yachts use these grills, so I assume they do have a rust-resistance to them.
There are several grades of stainless steel, with some containing more iron than others. One test for stainless steel is to see if a magnet will stick to it or not. That will tell if the stainless contains any iron or not. A magnet will not stick to my FEC100, but will stick to my PG500, which tells me they did not (or did not when my units were built) use the same type of stainless steel. Both of my units are stored in my unheated garage when not in use to keep them out of the rains, and neither show any signs of rust. I live in "The Great Pacific Northwet". FEC100 = 5 years old & PG500 = 3 years old. If you are storing outside, buy Cookshack's cover - they are well built items.