No electrical power - everything is dead?

Fuses check ok. Battery and alternator check ok. Still large voltage drop problem under load. With ac. Fan high. Lights on. Etc. What is this fusible link. How do I test.I am getting 12 volts to alternator with truck off. My research on the link shows that it will burn out during spike in voltage. Wouldn't that mean I would not get 12 volts at alternator with truck off? Thanks