Try disconnecting the positive battery cable and using an ammeter see how much current is being drawn with the ignition switch off. The radio memory and maybe some other small things draw current even with the switch off. If the draw is minimal (less than 1 amp) I say you got a bad "new Battery". It happens.
Be careful as most multi-meters are only rated for 2 amps or less in the current (amp) mode.
Also if you take it to an Autozone or NAPA store they will check it all out usually for free.
2005 Sun Voyager Workhorse W24 with 8.1 ltr Chevy. Allison 2100 transmission. Full body paint.