I just re-read your post and have one quick test to consider before any others..

Put the headlamps ON before you try to start .. if the headlamps go out when you get to starter crank position, you have a bad connection at bat or a low bat from sitting.
This is just a DIYer Bat LOAD test that does test power/load
You have 12 volts , but you may not have cranking amps [power]
Do this test before my othwer recommends .......