It looks like your Tube is built in Hypalon which is easier to repair. You'll need to start with the repair to the cone area before looking for other leaks.
Have you shipped any water in to the tube? You'll need to get this all out first and thoroughly dry.
From the looks of it you don't have a tape which creates a leading edge on the tube to cone join so you'll need to do the repair and leave it over night at least.
Do a search for Hypalon repair on here there's some good guides, if it is damp/humid where you are then try using a heat gun to keep the repair area dry and warm, a good job comes from well prepared surfaces and thin coats of adhesive with firm pressure rolling after.
I'm not sure where the seam is on these boats, it will either be behind the Strake or at the bottom where the tube sits on to the hull, have a look inside before you do the repair with a torch.
When you've done the repair check it's not leaking and see if you still lose pressure in the other chambers. If you don't then the chances are the bulkheads are leaking, this is not really an issue on older boats if it is slow, it's only when you can inflate the whole tube on one valve that you know you've got a problem!