For my 69 Safari I have completely removed every last bit of old sealant from the cracks and crevices... and really sealed it up good using a medicine syringe (the kind you give kids medicine with, not the needle kind) to pump it deep into the spaces. I sealed every window and flashing, all access panels, removed cleaned and replaced all the roof plumbing vents, stove hood and fridge hood covers. Repaired and replaced a broken wing window with Lexan and all that with not even two tubes of grey Sitkaflex. It cures rubbery so its not getting covered in dirt debris like some of the other sealants that remain tacky forever. For any overlap seams that let in moisture and around the door I used the aluminum tint gutterseal I purchased from vintage trailer supply. Can barely tell I have used any, the tube doesn't even look squished yet. Then I used captain tolleys on all the rivets and the seals around the windows that have shrunk with time and were letting water in, At first I used a 3mm hypodermic syringe to apply the captain tolley's to each rivet but it started clogging the needle so I gave up and applied right from the bottle. Clean up any excess with a damp cloth right away or you will have some elbow grease to remove the runs.
Just finished the front half of the trailer yesterday with the tolleys and hosed her all down top to bottom for 20 straight minutes this morning and the front is completely bone dry inside. Take your time do it right and you will be very happy in the end. Of course how tidy you are with your application and how thick you make your seams will make the difference with how much you use in the end.
So for me 1.5 tubes sitkaflex, 1 tube Aluminum tinted Gutterseal and by the time I finish every last rivet and the rear window seal I expect will need to purchase two small bottles of the captain tolleys. I purchased the Tolley's from West Marine here in Canada very recently