Sounds like wiring issues, things like bad connections at bulkhead connector, possibly regulator, or grounds. With engine running, should see 13.8V might deviate a few tenths, some temperature related.
Both 18V and 7V very bad, not something you want to drive like that...may damage electronics, wiring, battery or electrical fire.
If regulator is mechanical, upgrade to electronic. Also review the numerous prior posts, by 67Dart273 bout trouble shooting wiring and charging system.
