I'm no electronics tech, so, I will ask a couple of comments here, that MAY help you do a much more efficient system.
First, assuming you have an insufficient battery system, look up on Ebay, "Nissan Leaf Battery Module". They can be bought for under $90.00 each, (you would need 2) that would give you 15.2V nominal (working voltage) 16.1 V topped off, at 60Ahr each, so, paralleled that gives you 15.2V @ 120Ahr. nearly triple what that lead battery is capable of.
I have 2 different laptop chargers here, one is 15V output and one is 19V output, both from Toshiba Satellite Laptops. Check your laptop charger to see what it is rated at.
I tried a voltage test from a USB port and got 5.1V output.
My theory is, your car charger puts out over 15V (your info), so, that would charge the Leaf double module pack, and, run the laptop directly off the leaf pack and the cell phone off a USB port ? Cheap 12V inverters are VERY inefficient.
These Lithium batteries have a LOT more capability of powering your system, over a lead battery.
In EV car lingo, "lead is dead". A comparable lead vs lithium battery, you get 3 times the available energy from a Lithium battery and cycling is what these are designed for.
Thus, you run the car a lot less than needed right now. Lithium modules can last for years, not months, with a little careful monitoring.
If anything I presented is wrong, I am certain someone on this great site will enlighten me/us.