1994 Ford 250. 50K miles. Battery dies if truck not used for a few weeks. three batteries in last three years. Mechanic says trickle or intermitant short expensive and hard to find. Installed a disconnect screw on battery terminal. Works - most of the time. Battery saved, but occasionally starter won’t turn even when lights work. Battery terminals clean. Disconcerting when camping in the woods.
Ok, have you started by checking the relays to see if one is stuck in the closed position when it should be open? I’ve had family members where their relay was stuck closed and that caused the fuel pump to keep running and therefore drain the battery. It’s probably the best place to start.
I also forgot to ask how old the battery is. If it’s 5 years or older, I’d replace it.
Finding a parasitic drain can be a challenge but there are methods that you can use to help locate the trouble pretty easily most of the time.
It will help to have a meter that you can measure amperage with and you need to place it in series with the ground lead of the battery. You need to monitor the current flow while the truck is parked and nothing is turned on except the things on normally while parked. If you have over 80 milliamps of current flow then you start pulling fuses one at a time to see if you can cut the flow to a normal limit. Once you find the circuit that is drawing the excessive current then you can trace the problem down on it. Investing in a service manual for a wiring print is a must. By doing this repair yourself you can save on some shop repair costs. The savings on the repair will more than pay for the manual and the meter if you don’t have these items already.
If you don’t find the draw by pulling the fuses try disconnecting the alternator leads to see if that is the problem. Be careful if you do that though as the main output lead is hot to the battery, so disconnect the battery ground lead first.