To lose that much, my guess would be you are loosing one of the two power feeds through the ignition switch...

If you can find the schematics for your vehicle I'd check for a common feed point for all the stuff you are losing..

Suspecting the ignition switch itself since it is a mechanical device prone to this type failure. Really need the vehicle schematics though to determine.

You will see one main power feed dividing into 2 circuits that feed through separate contacts in the ignition switch.. One feed through the ignition powers everything needed to run the engine and the other feed for the remaining loads..