2003 Chevy Tahoe. If I turn car off and open cargo door, for even a few minutes to load something, the battery drains and I can't start without a jump. What is that?

Sounds like your battery might be going bad. Check all battery connections they must be clean and tight. Also check alternator output voltage. It should be around 14.5 volts to keep battery fully charged.

dandyoun is right on. If your connection to the battery are corroded, it will not accept a charge or a very slow weak charge and your running off the alternator. If you have some WD-40 and a small wire brush, and even a fine grit sand or emery paper, that will clean the connections to the battery. As long as your battery is disconnected, take it to a shop and have it tested. If there is one bad cell, it will not hold enough juice to turn over the engine. New batteries can go bad quickly, and harsh winter weather or hot summer weather can wreck havoc on batteries.