Let's see, 20 pounds is less than 0.5% of a 2 1/2 ton car. While it will make a de minimis difference in acceleration and energy used to get up to highway speeds, it's hard to imagine that it will make any measurable difference once the car is at cruising speed...which is where you'll probably be for most of your journey.
Probably more impact to adjust the cabin temperature up or down a degree and shut off the A/C when you can...or turn down the fan speed a notch...or turn the radio volume down one step...or pass up the dessert at the diner.
And, paying the SC to remove the liner and then put it back later will wipe out any savings on SC charging costs...especially if they're free to you. And don't forget all of those electrons that it will take you to make two round trips to the SC.
Have a fun trip!