Airstreams and other brands have what is often called a belly pan or belly skin under the steel frame.
These all aluminum pop rivets with extra wide flanges are great for securing the belly skin to the steel framing from below. The large dimensions will keep the soft aluminum sheet attached for years, even with road vibration. All aluminum construction means they won't rust with road salt.
Rivets are 3/16" diameter body with a grip of .250" to .375". If you are needing to clamp just two sheets of skin together, consider these smaller large flange rivets: VTS-2274 Wide Flange 5/32" Rivets
The flanges are 622" in diameter.
Ideal drill size is #11 bit. We have them here: VTS-122 Drill Bits for Vintage Trailers
Clamp your belly skin in place before riveting with our 3/16" clecos here: VTS-821 Spring Clecos
Install with a regular pop rivet tool. Bags of 100.