Hello thank you for using JA let me see if I can help.
The only reason that a vehicle will need to be jumped is a faulty battery. If the battery is fully charged and checks out okay the next thing you need to check is the battery post, make sure they are clean and tight good connection. Once the battery and the terminals are ruled out need to check the battery cables which with time will get corrosion and have a high resistance. Start by checking the connection and the cables. It cannot be the alternator by what you state above the alternator is charging and is keeping the vehicle running and thus is charging.