Just found this battery maintainer. I don't know how well it works, but it would be pretty convenient mounted permanently under the hood. Then just plug it in like a block heater.
https://www.crutchfield.com/S-8Cs6dUZdOKp/p_598GENM2/NOCO-GENM2.html
https://www.crutchfield.com/S-8Cs6dUZdOKp/p_598GENM2/NOCO-GENM2.html