
What I use whenever I have to put screws into the sheet metal exterior of my trailers. (Like for mounting vents, hose connections, cover plates, et al.) You can first screw them into a cardboard box and spray paint them whatever color needed to match your sheet metal.
#10 stainless steel hex-head screws (whatever length needed) with stainless steel neoprene cup washers:

Self-tapping (not my favorite due to slightly looser fit):

Correct pressure/tension set by adjusting the torque on your screw gun:
https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcRJvcui3hboDFAlNHQYp480dD0l-GXygvAXBFzzyLb0Gesz_5OyThe painted/electro-plated zinc version of these are okay, but I just don't like corrosion and rust, so I only use SS.

My goal...
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...is to live in a trailer.