[mr48chev]

From 50 years of experience, "new" or "freshly rebuilt" doesn't always mean that a part is in perfect shape and functions perfectly. Still if it is on the keyed circuit that should be good enough.

As I suggested, check for voltage running across the top of the battery to make sure you don't have a drain there. It doesn't take a lot of dirt and moisture to make a path for electricity.

Other things I've run into are a clock in the radio that draws more amps than expected and drains a battery over a few days. I've seen brake light switches that will stick on or pedals that will drop enough if they don't have a return spring to let the switch turn the brake lights on. interior lights switched on and forgotten especially if they don't put out a lot of light to begin with.
I had one student who didn't take his custom 64 Chevy to work at Safeway at night because he didn't want to get it dinged in the parking lot and left it parked at his house. Every morning the battery was too low to start the car and he had to jump it to drive to school and we spent a number of hours checking every part and piece of the electrical system. About that time he was fussing about it to his mom and she says "Oh, your brother and his friends stand outside and play your car stereo in the evenings. A bit of a heated discussion with his 13 year old brother cured the problem.