Maybe you could try using a power inverter. You hook it up to the battery and it inverts the power in your vehicle to 120 volts. You can get one of these from any truck stop or even Wal Mart for that matter. Find out how many watts you need to run the bater in you vehicle before getting one too...
It is best not to rush the process. Day 18 is only relative since we as humans dont feel the vibrations when they are ready to pip like a broody that knows exactly when to stop turning them. So we use day 18 as a "happy medium".
I dont think there is anything you can do. I had a couple do that from my last hatch and they just drug it around til it eventually fell off and the are doing great.
If you have no pips, you should definately do something to get the humidity up to as close to 75% as you can. If the humidity is that low when they pip, they could shrink wrap anyway.
I try not to get below 50% or higher than 65% until lockdown. Then 70% - 80%. I live in KY and that seems to work great for here but percentages may varyby location. The main thing is that the eggs have to loose some of the water inside it so that during the first 18 days they loose 11 to 13...
I just built this bator and this is the first batch the come from it. I always let shipped eggs sit in room temps for 24 hours before putting then in the incubator. This allows the eggs to adjust to air preasures that change from shipped to destination locations. I dry incubate for the first 18...
I had 12 out of 13 cream legbars hatch yesterday. So far 1 rhodebar of 6 with one pip. And 1 pip of 15 marans that were set 2 days later than the creams.