Losing electricity

When I start the 2005 Dodge Magnum (I think, 3.5liter)V6, all is well. In my
garage (about 40 degrees), 2 days later, I go out and it won't start. I jump it
at the battery under the truck, 15feet away from the starter, I checked the
alternator, that is about 14.6v (don't actually remember, but it's good). I
bought a new battery, all seemed well. Now, 2yrs later, it's back to the same
thing, after 2 days sitting, it needs to get jumped. I have no auxiliary stuff
plugged in inside the car?
...Maybe the cables are bad?
...What could be wrong?