You should not need to replace anything with this type of problem. It is most likely a loose connection.
I would test the batteries firsts. A load test will show if one is weak. It is possible to have a weak battery and still see twelve volts on a meter.
Then check all the battery cable connections. Look for loose or corroded ends. A bad connection will have voltage but, it will not carry the load for cranking.
That is why you can try the key on and have power but, as soon as you try to start, everything goes dead.
If the cables look swollen, it could be damaged internally. Any cable that has damage to the insulation or connectors can allow corrosion to build up internally and the cable cannot carry a cranking load.
If you have automotive repair ends on the cables, these often cause poor connections and cranking problems. I replace these cable with factory cables when I find them.
If your cable and batteries are good, check in the dash for connections at the breaker and main relay. Focus on the large red and purple wires first, then look for other loose or corroded connections.
I will be away later today but, can help you more this evening. Just let me know what you find and I will be glad to help you with this problem.