Depends on how how you are going to support the plate. I used 1/8" diamond plate for platforms of side rails and to make a hatch cover, it worked fine for the side rails but only spans about 12 inches. I sleep on the hatch cover, which is 19 inches wide (unsupported span 18"), and I've noticed it's sagged a little bit - I'm 175#.
An old floor made by PRO - pretty much the original frame maestros, at least in the 80's and 90's - has also sagged a bit over 20+ years and it is 1/8" dp supported by 1" EMT, but has spans of 16 inches.
So for a floor, I'd go to 3/16 at least, and support with 1" pipe or similar. Actually, for floors I prefer HDPE (Marineboard or similar) but it's heavy and expensive. Diamond plate is bomber when done right, and pretty forgiving for sloppy weld(er)s if you support it properly. It's hard to get paint to adhere (rattle cans) but you don't need to paint a floor...