Method1. Clear or white external mastic. Do both sides of rails if that's where it's coming in. Re-do in 12 -15 months.
2 purchase the caravan none setting sealant/putty and screws if they are rusty
Remove the awning rails on each side, clean off the existing putty/sealant dump it.as you apply the sealant/putty put a match in each of the screw holes as you go so you can put rail over match pull out match put in screw keep lose, you will find each scew hole easy.
Lasts for yrs do it properly.