With no electricity, gas can not be pumped from the tanks.
Even when it can, there is immediately a line-up of 100s of vehicles.
(I see this very often in West Africa)
I'm guessing that if the number of electric cars surged by 25% for a few days, it would be much less of a problem.
[0]: http://www.kgw.com/news/eclipse/gas-shortages-long-waits-rep...
Some parts of the US aren't able to easily meet energy demand they can forecast when it goes up by amounts like 25%. For instance, Pepco pays to install thermostats where they can turn down/off your air conditioner[0]. Same with Dominion Energy[1]. I would be unsurprised if an influx of people of the size to cause gas shortages would also cause problems for electrical grids in many parts of the country.
[0] https://energywiserewards.pepco.com/md/faq/index.php#q2
[1] https://www.dominionenergy.com/home-and-small-business/ways-...
My company maintains an old boiler from the 1880s connected to a steam engine/generator from the 1920s. Because we have it maintained and can start it up with 12 hours notice the power company gives us a substantial discount on our power. I understand we actually did turn it on about 5 years ago when something major happened to the grid. We were able to power our [small] town alone. This is one of the least efficient power plants in the country and thus one of the most expensive.