Do you want to caulk just the seams, or surface coat the roof? Either way, the present materials will have to be removed, before any new material is applied. Here's some info on good caulking materials:

http://www.horsetrailerworld.com/forum/thread-view.asp?threadid=10519

Another alternative is the application of an Elastomeric roof coating. These rubber based coatings feature a high reflectivity, a high resistance to UV damage, and feature very large elongation and strength properties. They work well on metal roofs forming a completely weather proof membrane. They are somewhat expensive, but can be applied by a homeowner using common painting tools. There are several companies that make and sell these products.

Gard