Hello Toby -- Welcome to the Forums! You will see a variety of rivets used on your Argosy. You are correct that for putting back trim and all interior finish applications, standard pop rivets were used originally. Be sure and buy the pop rivets with the aluminum shafts -- not steel. Now... I have a '74 Argosy and can tell you that the Versailles, Ohio, plant used an annoying number of steel shafted pop rivets.
The shell exterior was put together with bucked rivets (solid) -- the large dome shaped ones you see over the majority of the body. The proper substitute is to use Olympic rivets (see link below). Always remove the rubber gasket if the Olympic has one -- most sources know not to send you that type -- the rubber shrinks and always loses the seal. Olympic & pop rivets used on the exterior should have a dab of Vulkem or Sikaflex 221 on them before you put them in the hole. Silicone caulks are a no-no and to only be used on houses with foundations! (leaks like a sieve when used on aluminum)
The belly skin needs a wide head rivet, so check out the Large Flange Rivets at Vintage Trailer Supply
. You'll probably find some areas of corroded aluminum and will need to cut out custom "washers" from some aluminum sheet stock -- I used 0.032" and it's plenty thick/stiff.