That's not technically wiring "in series". Wiring "in series" means that the output (ground) leg of the first device (USB port) is wired to the input (hot) leg of the second device.
I think what you're talking about is running 1 hot wire from the fuse block to the 1st USB port's input/hot leg, then daisy-chaining a wire from the 1st USB port's input/hot leg to the 2nd USB port's input/hot leg. (Technically this is wiring "in parallel".) That's perfectly fine, as long as the 1st wire (from the fuse block) is large enough gauge to handle full draw from both USB ports.
And, you don't need to run a ground wire all the way from the fuse block area to the back of the truck. Both USB ports can be grounded to a nearby metal bolt that threads into the body of the truck (with paint removed).