When I pulled the coroplast down on our SOB a few years ago, I expected those rusty screws to break. Not a single one did!. I did replace them all though with new ones from McMaster-Carr (
https://www.mcmaster.com/)plus a box of fender washers
If the screws do break off, grind the remaining piece level with the frame then install new, self drilling screws right next to the old hole. I, personally pwouldn't use anti-sieze though as it cab be (is!) real messy while woting under any RV but it is an option if desired