Frank do you have a ampmeter of some sort?
With the truck turned off just like you were going to leave it overnight. Disconnect the positive cable from the battery and measure the current flow. Start with a high range and then work down until you have an accurate measurement. That way you should be able to know exactly what you are loosing.
Then with the meter still connected, disconnect one electrical device at a time until one of them makes the current flow go away. That one is the problem.
Best to approach it with some sense of structure, perhaps pulling a fuse, and if nothing changes assume everything on that fuse is okay. Remember that some circuits are not fused on an old truck, and your problem may be something that doesn't have a fuse. To find those located each junction, like at the starter, and disconnect each wire while checking for the current flow to go away.
Before you buy and install a disconnect switch, have you tried disconnecting the battery overnight to see if that affects it? Could be a bad battery...
Where in the Seattle area are you? I'm in Kitsap county.
