There are several videos on YouTube on the topic. I found them with "seam sealing an Airstream" as search term.
I did it with standard grey vulkem, fast curing formulation.
Remove any loose material with a dental pick (available at Amazon).
Use a painting tape to run along BOTH sides of the seam to be sealed; leave only an 1/8 to 3/16 inch right where the seam is.
Lay down a small amount of vulkem right over the seam.
Use a rubber-gloved finger to go over the vulkem, pushing it into the seam and flattening it.
Pull off the two rows of tape.
Let the vulkem cure.