If you are sure you have set the multimeter probes into the correct jacks and you are actually only reading one (1) Milliamp, that is static electric in the air from you or the meter or the wind or outer space. Start by suspecting a bad battery or a bad charging system. Check battery voltage with cables off and see if it reads 12.6V. You probably just have a dead battery.
I really can't believe you can measure 1 ma. Also a "1" is displayed on some meters to mean "open circuit".
A reading of 1 ma is ZERO drain.
ESD can be measured in amps or volts using a multimeter.

Hope you do not have some fancy modern thing in your truck, that you forgot, and has a light or hold up in it as Shakey said. Anything that will display time or date has a hold up. Theft system has a hold up. Music/USB/GPS/radar detector ports may have drain.


Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn