Battery keeps going dead, also no start issue

Last Monday my truck wouldn’t start due to a dead battery. for some reason even with the key not in the ignition the battery was drained my my iPhone being plugged in it jumped right away no problems for two days. Two days later with nothing plugged in or any lights left on the battery was dead again (I thought). All the interior lights and radio would come on and work just fine but when I turned the key nothing happened , no clicking , no cranking , just nothing. I tried jumping it and found the positive terminal was loose so I tightened it and eventually it started and worked all day (turned off and started back up fine multiple times). The next day, the same thing happened; all the lights and radio worked but nothing would happen when I turned the key. Tried jumping it and it started right away. I went to my uncles to try and clean up the terminals a little and it worked for a little while. I had it off while working and it started back up just fine a few hours after cleaning the terminals... a couple more hours later , I tried to go home and it wouldn’t start ( same situation all the lights work , radio, but no clicking or cranking). tried jumping it , this time it wouldn’t jump, tried putting in a different fully charged battery and that made no difference either. also for some reason if I haven’t already alluded to it , something is draining the battery when the key isn’t even in it. not sure if that is relates but it seems like a short or something but I have no idea where to even begin on this and unfortunately I only have a few days to fix it or I’m SOL. Any help would be greatly appreciated. I am currently not with the vehicle as it is stuck at my uncles but I’m trying to get back over there today ASAP.