Put the battery on the charger for a few hours, then remove it.
What battery voltage is it showing?
What is the voltage from the battery - to the valve cover?
What is the voltage from the alternator + to the battery - ?
When it cranks, does it sound strong or is the starter struggling?
Pull the spark plugs and take a picture of them lined up cyl 1-4 with no flash with a white background.
Have you checked all your fuses (both in-cabin and on top of battery). Test from battery - to both sides of the fuses to ensure all voltage look about right.
Go through the bay and look for any cut or frayed wires.
These are all good places to start.