Follow the positive battery cable back, from the battery. There should be that circuit breaker within a few feet. Yes, it's sometimes in a junction box, tucked inside the frame. If it has a button, it's a manual reset breaker. If it is just a metal can, it is self-resetting. Replacements are available at auto parts stores.
How are you determining your battery is good and charged fully? Do you have a hand-held meter?
If your converter is not outputting charge current, it may have a fuse blown. Check the 12V fuse panel or on the converter, itself. These fuses will typically be higher amperage--40, 30, or 20 amp fuses. Again, available at auto parts stores.
