To get the I Got You A Pack Mule achievement in State of Decay 2 do the following:
Rucksacks are the big backpacks of stuff you can find out in the world when scavenging. There are different types based on what kind of resources you've found, though this achievement doesn't require any specific kind -- they all work. The fastest way to do this one is to bring a truck that allows six trunk inventory slots and fill it with rucksacks as you go.
Six in the trunk plus one on your back means you'll only need to make a few trips. Pack them up and bring them back to your camp, storing them in your storage facility (the game will show you where to go). Note that long scavenging runs like this will be costly to your other items and degrade your weapons, so pack prepared and scavenge for the little things as you go too.