Do NOT use silicone on your airstream! Silicone corrodes aluminum. I'm not sure what airstream uses for a roof coating, nor do I know if they ever coated roofs. New trailers have a white fiberglass panel for the roof.
For caulking, use Vulkum (aka trempro). The gray color matches well. I think it's 137 or 136. It's available from Vintage Trailer Supply, InLand RV, Out of Doors Mart, and other places. I've read where it's also available at Home Depot, although I've never looked for it there.
For the white roof, can you figure out if it's just white paint, or a roof coating? A coating will be fairly thick. If it's a roof coating, then it probably needs to be resealed. Kool Seal is a brand I've used. It's white and self-leveling, and will stick to just about anything and seal it.
For sealing seams, use ParBond or Siliflex (SP?). I've used ParBond, and it flows into the seams pretty well. Others swear by Siliflex. Both are available at the same 3 online places I mentioned above I believe. Also, if you have a local Airstream dealer, it's all available from them too.
Best of luck tracking down the leak. They are really hard to pinpoint sometimes, so go over the outside with a fine tooth comb, and seal anything that looks suspicious. Use vulkum on the biger joints like around windows and doors, and ParBond or Siliflex on the exposed seams.