Constant random battery drain

Hello KH, I have looked through many reviews, pages, reports etc on issues related to what seems like an instant random power drain from the battery in my vehicle listed above and this is actually the first time I have ever felt completely baffled and your in depth replies and process of elimination has actually brought me to create a account in hopes of picking your brain, I’ll give a run down best I can of events and attempts at diagnosing my issue, oh and it has 300,000 miles on it and before this recent onslaught ran wonderful minus it leaks oil, I bought the vehicle in Oklahoma Just to drive 1,400 miles to Ohio and it shut off on me one time when I was braking to stop before I made the trip, started right back up and when I got off the road I checked under the hood and the ground battery clamp was loose so I tightened it and that was it as far as it shutting off on me until the recent onslaught. I ended up continuing to drive it for about 2 months and it happened again except this time it bogged down
Right after heading up a hill in the road as if it were out of fuel and unable to increase torque ( like trying to go up hill in 5th gear in a standard) and when I got stopped turned the key back on it’s as if the battery completely emptied out of left field, click click click buzz no go, I waited about five minutes tried again and started up as of it never had a problem, That was in October, I left it with a Marine I served with for a few months and he drove it pretty much all winter said he never had a problem until one day he did and said it just went dead all the sudden and not enough juice to start it, He got it back to the house and charged and tested the battery, it checked out okay so put it back in and it drained out this time by the time he got to the end of the driveway, So he figured it was the alternator, And it sat until I came back in May, He told me about it, we put another battery in it we knew was solid started right up I let it run for about 20-30 minutes and was gonna take it for a spin, soon as I put in gear it acted like someone stalling a standard trying to take off, Turned the key off turned it back on and click click click buzz as of it was a all but dead battery, so I let it sit, for a few a bit and charged both batteries full checked me out they were good cleaned terminals checked fuses etc, wires, finally put the battery back in started right up, started right off to take it for a drive and made it 10 ft before it drained, same kind of way, jumped it, parked it again let it run about 20 minutes turned it off turned it on no problems, couple days later started it let it run about ten minutes shut it off, and did that for about week and a half 2 weeks, Every 3 days about, It held charge read full wasn’t draining so I check fluids and everything and finally gonna go drive it and as it was warming up running about ten minutes started bogging down and just quit, Had just put 3.5 gallons of fuel in, Battery was dead again. Alternator reads that it’s charging and heck one time when I jump started it I didn’t leave cables on just let it sit for five minutes and then the battery read full and stayed that way until I went to leave. Any help would be much appreciated sir.