Well first don't use that 30amp fuse, I doubt the chargers internal wiring could handle 30amps, so the wiring would burn up before the fuse blows.
If load is 3amps then use a 5amp fuse.
I would hook it to the cigarette lighter circuit, it isn't a Key off circuit, but I think it might be a better use if you can charge devices when key is off, I doubt anything you could plug in to this charger could use enough power to drain a 400amp battery before it was fully charged.
But if you want to use a Key on/off circuit you can look in the fuse box in the cab, get a test light or volt meter.
Many of those fuses are only active with key on, you can buy inserts that allow you to tap into a fuse circuit in the box.
Pull out the fuse and then turn key on and off, only one side of the fuse holder will have power with key on, that is the side you want to tap, install insert and then put fuse back in, hook your 5amp fuse to the insert and to the USB charger, ground the charger wire to existing Ground Point, there are several on the firewall or near the fuse box.