Tubes made with natural rubber (if they are even still made), will lose air over time, although it usually is (was?) much slower than your experience. I'd go with the bad valve stem, as noted by others, as the likely culprit. Oh, also just check that the little "pushy-thingy" thing inside the valve stem is tightly screwed in - very occasionally one of them will start to loosen up and start seeping air. Back in my wild days, a couple of times I unscrewed one of them, at different times, on wheels of cars that were especially ignorantly parked and blocked access for others, etc. Caused great inconvenience to the offender without actually causing physical damage.