While recently replacing a damaged (by me) side marker light assembly, I had difficulty removing the assembly's two mounting screws. These screws were badly rust 'caked' and were the same steel screws used inside my coach. So I used stainless steel screws instead when mounting the new replacement assembly.
I suspected these same 'interior use' screws were also used for external attachments throughout, and set about replacing ALL of them with SS screws. I wanted to head off future maintenance/removal problems that would surely occur. Then it started!
I began by attempting to remove the driver's side tail/turn/brake/emergency/backup light assembly's screws. I was immediately again faced with difficulty removing all six of the same internal steel mounting screws. They were all rust caked and one screw even broke off deep inside the coach wall. The effort to replace all six screws with SS ones took about 90-minutes for what should have been a 15-minute job. The 90-minutes included carefully drilling a new screw hole beside the original mounting hole blocked by the broke off screw deep inside the coach wall.
So - with an interest in future maintenance - I recommend everyone consider replacing external mounting screws with SS ones if they're not SS already. The risk in not doing so could very well make removing assemblies for maintenance/replacement in future impossible without destroying the assembly, and such rouble would be compounded by broke off screws. My Prism motor-home is a 2015 year model and already has rusted out external screws.
You can buy 40 1" X size 8 SS screws at H. Depot for about $5 ... less than 13-cents each!
Shame on FR for not using SS screws for external applications in the first place