If not the alternator what else could it be

If you find that your battery is dead after sitting a few days the first step is checking the battery. Most auto parts stores will perform a load test and alternator test for free. If those check out then you have a current draw. A tech will start by hooking up an amp meter and pulling fuses one by one to find which circuit is drawing. It's most likely an electric motor or module that is faulty and staying on. Use this link to find a good shop near you. https://repairpal.com/repair-shops Alex