Basic electronics....
On a 12v system everything will have a 12 volt power source unless one condition is encountered, resistance. It will drop voltage plain and simple. It is a law of electronics, period.
To drop down to 9v at the 30 amp fuse means one of two things:
1) Reading is wrong, retest to verify.
2) Usually corrosion is built up on the connector and/or inside the wire that increases the resistance. Look for a whitish powder or a greenish appearance on the wire / connectors. Clean as necessary - clean and bright is right.
(We had this on a few buses that had low cranking speed)