Starting problem

[jwf54]

Not quite sure what you are driving at and which "starter cables" you are talking about.

To test the battery you run a pair of jumpers directly to the starter motor.
The - to a good earth near the starter ( oil drain plugs works well ) and then from the battery + directly to the + on the starter motor
It works better to connect the + first and do the earth second so you don't accidentally directly short the battery.

IF the starter turns, the battery & starter are good to go.
If not then repeat with a known good battery , works, then you battery is toast, dosn't work then the starter need work.

As for your funny readings, a direct short will cause a 0 V readings as will accidentally reading between two + terminals.
Batteries store energy or Watts if you like.
Watts are V x A .
So as the amounts of Amps the battery supplies increases the voltage it supplies these amps at decreases, what is commonly called load voltage drop.
This is why your headlamps go dim when you start your car with the lights on.

Ran Jumper from starter to battery and got nothing. Took my RV battery that I had on the bench and jumped to the starter and ground (earth) and the starter turned over. This leads to the battery being the problem but I was still getting a 12.3 V reading with a voltmeter.

I pulled the battery out of my mower and took it to "Batteries Plus" store and they put it on their tester and it showed 8V then 6V bouncing around. The guy said that it was probably a cell going bad and what I was measuring was a "surface charge". When his tester put a load on the battery the voltage dropped, which agrees with your Watts= VxA discussion above.

I bought a new battery and dropped it in and the mower started right up, problem solved, THANKS!

I sure know a lot more about my mower electrical system than when i started. BTW...do you know how I could have tested that battery differently to load it like the guy at the battery store did and measure the voltage without having to buy an expensive tester?