How could this be done?

[Dead Parrot]

Parallel is a bad idea. If the flashlight is near dead, you would get a very large initial current from the truck battery into the flashlight. Lead acid batteries can both put out and accept large amounts of current, more then enough to melt the average non-battery cable wire. Why they are still used after all these years to power the starter.

Sounds like what you need is a DC-DC voltage converter powered by the truck battery. Feed the output of the converter into a current limited charger for your flashlight. Make sure to include a timer/auto shut off or you might need a jump to the the truck started.