The only REAL way to diagnose this is with a volt meter. Start at the source, the battery. Put the black wire of your volt meter to the engine somewhere metallic, such as the alternator housing, or cylinder head. Put the red wire on the positive battery post sticking up from the battery. There should be 12.4 - 12.6 volts. Next, touch it to the terminal that is bolted to the post. It should still read exactly the same volts. Now climb under the vehicle and touch it to the large terminal at the starter. STILL should be around 12.3 to 12.5. If the number dropped more than 0.5 volts, (1/2 a volt), then you must start by cleaning ALL of your battery posts and terminals by removing the clamps and use a wire brush on the terminal posts and a terminal cleaner inside the round part of the terminal clamps. Battery terminal and post cleaners are cheap at Napa, Oriellys, Walmart and Auto parts stores everywhere. The reason I just spent so much time writing about this is because EVERYONE says, "nope, they're clean" without even checking them and 75% of no-start issues start with poor connections. If this doesn't work I will keep going.