Airstreams and other brands have what is often called a belly pan or belly skin under the steel frame.
Pop rivets with extra wide flanges (these measure .467" in diameter) are great for securing the belly skin because the softer aluminum sheet often has holes that have gotten oversized from old fasteners. The large dimensions will keep the soft aluminum sheet attached for years, even with road vibration.
We sell 2 sizes of wide flange rivets. This is the smaller 5/32" diameter size with a hard-to-find clinch range of just .020" to .120". That very thin clamping range is perfect for clamping two sheets of aluminum together when you are not attaching to a steel chassis member. If you are clamping sheet to steel chassis frame, use the 3/16" rivet here: VTS-283 3/16" Large Flange Rivets
Ideal drill bit to prepare hole is a #20 available here: VTS-122 Drill Bits for Vintage Trailers
Clamp sheet in place before riveting with black 5/32" clecos here: VTS-821 Spring Clecos
All aluminum construction means they won't rust with road salt.
Install with a regular pop rivet tool here: VTS-272 Heavy Duty Riveter
Bags of 100.