Daniel;

You have not given very much background info to go on, so there are several possibilities...

Do you have clean and tight connections on all connections having to do with (at least) Charging System, Battery, Starter, Ampmeter, Ignition Switch, and Fuseblock?...because poor connections can manifest themselves in weird symptoms including your apparent quick (overnight) discharging of battery (because it is not fully charged to begin with!)...this might even be suggested by the pegging Ampmeter at high RPMs, which suggests battery is not fully discharged and current is flowing only under highest voltage output from charging system condition...

A "58 pv444" with its 6V system, has half the voltage of more recent even vintage vehicles, and can stand to loose even just a little of this voltage even less...because of this, it makes having clean and tight connections even more critical to a reliable electrical system.

Battery discharge can be caused by a load external to the battery (which is not being disconnected when turning OFF Ignition), or even an internal issue (pretty uncommon for a brand new battery but not unheard off)...but are you certain you are starting with a fully charged battery?

Check voltage to be certain battery is really fully charged!
System voltage measured right across battery terminals should be above 7 Volts with engine running.Make certain that you are starting with a completely charged battery...charge it overnight with a line powered charger...the on-board charging systems are not intended to charge the battery from completely or nearly completely discharged...they are intended to run the vehicle, and have a little left over with which to put power which was taken out by starting back in and then continuously trickle charge it (and at that rate, it would take them days to charge it from completely dead!)...and further, they wont do this very well at all with poor connections in-line!

Let us know how you make out!

Good Hunting