Start at the beginning!!!
Make sure the fuel is off.
Turn off the shore power and try to start the engine---NO START?
Check the battery voltage at the battery posts (not at the terminals) 12+ volts
Turn on the shore charger power again and check voltage at the posts--13+ volts??
Check at the terminals--13+ volts
Yes!! Clean terminals and allow the battery to charge.
Battery has been charging for hours but when you turn off the charger the voltage dropped to-- 10 1/2 volts.
Yes!! The battery is toast---replace.
I have two (one red and one black) 14 gage jumper wires that I can conect to make 20 feet if needed.
Conect the jumper wire to the cable end that attaches to the engine and the other end to the black wire of the meter. place the red meter lead to the battery post.
Yes--You have just tested the ground wire to the engine and found it good.
No voltage--ground wire broken or coroaded--fix and try starting--No??
Now a leap of faith--
take your meter to the engine but first turn the batter selector switch to what ever position you normally use to start the engine.
Conect the black wire to the place that you checked the ground to be good and the red wire to the big red cable to the starter, or the starter solonoid (if remolt)--12+
Yess!! You have just tested the entire system(battery switch, and all conectors to this point and found them good.
No?? Using you engine ground, use the red meter lead to check the positive lead from the battery to the switch lead and then the cable to the post. with switch in the on position check for voltage on the out post of the switch and then from the post to the cable and then from the cable to the starter post and fix what ever is corroded or broken.
Voltage is now good but still won't start??
Locate the solonoid trigger wire ( it the little wire) and have someone turn the key on and operat the start button or start side of the switch
Check out the pannel.
You might want to rig a jumper wire from the big feed line and touch it to the trigger terminal on the solonoid and if the starter works you have varified that the problem is in the starter switch.
Well you are getting close to the end.
You have power to the starter solonoid and triger power to turn it on.
Turn the switch to start and check for voltage on the starter side of the solonoid 12+ volts??
NO?? Problem is the solonoid!!
Yes?? Problem is the starter!!
WAIT JUST ONE MOMENT!!
YOU FORGOT TO CHECK THE GROUNDING OF THE STARTER!!
Probe the case of the starter and make sure it has a good ground. If not make it right before the above assumption is correct.
Honestly, I am betting on the battery being bad.