((Careful - a tongue jack can pull 20-30A under load. This is more current than the 12V vehicle-trailer wiring can handle. Obviously this shouldn't be an issue since you're LOWERING the tongue, but it could be if you overshoot or decide to adjust it back upwards.
Might it be easier to simply pull out the manual crank for the jack and not bother with the motor, if you're only dropping it a few inches?))
Not to be smart, but I don't recall hearing of this ever being an issue before. Many raise and lower their jacks all the time pulling in and out of campgrounds daily while plugged into the vehicle wiring. I have done it for years on multiple vehicles and can't say that I ever had a problem that I was aware of. Being that the truck wiring is on a fusible link anyhow, I would want to think if there was a potential dangerous excessive load draw on the wiring; then the fuse protection should kick in. I see what you are leading to, I just would think the only concern would be to continue to extend or retract the jack after it's physical distance allows causing an excessive draw.
