Was driving and quit on me and cranked slow so towed home battery tested said dead cell and bench tested starter using a screwdriver to simulate a load on gear and and spun fast with driver jammed against gear. So replaced battery but still turns over slow and when cranking drops from 12.6 volt to 11.5. Help am at a loss as to what it could be

 

Clean all your cables and grounds.

 

Already tried that

 

If you replaced the battery and checked/cleaned all battery cables and grounds, then the next logical item in the chain is the starter. I'm not sure how accurate is a bench test you describe with the screwdriver jammed against the starter... Perhaps take your starter to an auto parts store and have them run it on a proper tester?

 

Auto parts can only bench test with no load

 

Ok, then different approach:
hook up a voltmeter across the battery "+" and "-" terminals. Note voltage. Crank the engine and note battery voltage while cranking. Repeat this process by measuring voltage at positive terminal on the starter and negative terminal on the battery. If you have significant difference between the two measurements (at battery vs at starter), you have an indication of bad wiring or bad ground somewhere. You then can try to locate the spot that produces the voltage drop in the circuit. Fix it by cleaning connector or replace the bad positive cable or bad ground strap. If you have similar difference in voltage at starter as you have at the battery - you have a bad starter.