caution is warranted
I'm making a general point of caution as I don't know the details of some of the previous posts in this thread. Not trying to start a debate.
Screws that attach fuel tanks in RV designs are structural fasteners. Changing the screws to something besides what the drawing specifies should only be done after thorough technical analysis/consideration. In general (that phrase again), stainless steels do not have the same strength as carbon steels, especially aviation grade. It is highly probable that some design margin has been lost if you do change the aforementioned materials. The quickest check would be to compare the shear values, if available, for the two fasteners (equal diameters). Just sayin'.
BTW, As to the original post; my two cents = don't paint them. If someone looks and doesn't understand, no biggie. You may get to educate/impress them on integral tank construction.
Build/Fly safe.