Have those of you with charging issues ever thought about just using one of these?
This one is expensive but the quickest I could find on the internet (I have a similar one that I paid $30 for).
Basically just plug it in and leave it overnight to charge your device when it isn't receiving charge from the car. You could also leave it plugged in to the tablet and leave it inside the glovebox. That way it keeps your tablet always charged until the charger's battery runs out. Then you take the charger out and plug it in overnight. This way you cycle your tablet's battery less.