If you've had pulpitis for years would the outside of the tooth be destroyed as well - would there be severe visible decay?

Not necessarily..: Pulpitis is inflammation of dental pulp tissue. This can be caused by a number of factors, including a deep restoration or even parafunctional habits (bruxism). A perfectly sound tooth with no fillings or cavities that is traumatized by blunt force or extreme temperatures may have pulpitis...

Pulpitis: Pulpitis is an inflammation of the tip of the root, typically coming from inside the pulp chamber. Most commonly it does start from decay, but not always is there obvious decay visible. In rare occasions, it could come from the bloodstream and lodge into the root and there may be no decay above the gumline whatsoever.

Pulpitis: Pulpitis is inflammation of the soft tissues (blood vessels, connective tissue, nerves, etc.) within the confines of a root canal. With swelling comes pain because of the restricting root canal walls. May be due to decay, trauma, fractured root, adjacent infected tissues, etc. For best answer as it pertains to you, see an Endodontist, a Root Canal Specialist.