Lots of good info so far. Having recently installed a pigtail on my battery terminals so I could easily run a trickle charger over the winter, I know how easily one can think you're getting the battery cables tightened down properly when you're really not.
The cables clamp down on the battery posts by means of a bolt on the top that goes down to a v-shaped piece under the clamp. As you tighten the bolt, it pulls the v upward and the narrowing of the v forces the two sides of the split clamp together. If you have accidentally caused that v piece to be a little off center, it will do no clamping, and even though you are tightening down the bolt on top, the cable will still be loose, so could make poor or intermittent contact. To fix this, you simply loosen the bolt and re-position the v part so it is centered under the split clamp and re-tighten.
If in fact some electrical item is not shutting off and is causing a parasitic drain, you can measure this with an ohm meter placed in between a battery cable and the battery. In other words, you disconnect a battery cable and put one of your two meter probes on the now disconnected battery cable, and the other probe on the battery terminal. The meter will show you what is being pulled from the battery by the car. It has been a while since I have done this so I can't remember if it has to be the positive cable but it might.
One expects some small amount of parasitic drain to be present to keep the clock alive, etc., but there shouldn't be so much as to kill the battery in 2 weeks. The last two winters I did not put my '16 SS into transport mode and I did not put the battery on charge; I simply parked it for the winter, and it still started up for me in the spring.
This winter, with the battery now approaching 3 years old, I decide to add a trickle charger. Based on posts in a battery charger thread on this site, I bought one of these
https://www.amazon.com/dp/B000FQBWCY..._hUlmCb0YAGFW2 because it was supposedly like what GM sells as a compatible charger for our cars.