1984 ford f250 inline 6 Have replaced battery, alternator, starter solenoid, starter, and voltage regulater, along with battery cables. Battery is being drained when the truck sits more than 15 hours. Don't have any idea what to check next.

If you don't have any idea what to check next, maybe now is the time to take the vehicle to an auto electric repair shop that specializes in tracing wires. You may have a short to ground somewhere, or an accessory, that is drawing current when the vehicle is off, to the point of making the battery go dead if sitting more than 15 hours.

One check that you could do yourself, with the proper tool, is to check the amps being drawn when the car is off. If the amps drawn is above .2 Amps, then it's too high of a draw. The symptoms you describe sound like it is more like 2 or 3 (or more) Amps being drawn on the battery.

Try to remove fuses to various accessories to see if that reduces the amp draw on the amp meter.
I would use an Amp meter that is inductive and has a claw that opens and surrounds the positive or negative battery cable and measures current without having to actually remove any cables from the battery, and connect in between the battery terminal and the battery cable.

Let's say, for instance, that it was an accessory like a radio that was drawing an inordinant amount of electrical current. Then, if you removed the radio fuse, the draw should go from 2 or 3 amps down to below 0.2 amps. So, that's what I would do to try to narrow down the possibilities.

Good luck on this repair.