New alternator and new Battery but why my Nissan Vanette still discharge after 2-3 days?

We bought a second hand Nissan Vanette (1950 model) last month. One morning when we were about to use it we just realized that the brand new battery we bought was already discharged. We brought it to the Mechanical Shop and after 5-days, the mechanic said that they have done and fixed everything that would possibly be the cause of discharged bu still the same problem was not fixed, they advised us to change the alternator and so we did. From there the Vanette died as they have already replaced the alternator with brand new again, and so they said we need to recharge the battery because it might be dead already. After doing it so. It worked and run for a couple of days, but as my husband tried it again this morning. The battery is dead again. Another thing we, just realized that even the aircon is dead. It's really hot inside the van. Please help us solve this problem, It's almost Christmas and we will be using it out of town to visit my parents.

thank you for the big help. Happy Holidays!