Started doing this with minivans years ago, I have thought about trying to form a business and even tried to start an LLC but don't have a business address so I'm waiting until I move.
The Renogy DCC-50S is made to go in a vehicle and use the alternator and also manage solar and house banks. Supposed to be used with an RV, but as soon as it hit the market I got one for my minivan and now I've moved it to the truck.
DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT
I would highly recommend even a very low budget form of this type of thing... even with just a cheap inverter and one battery on a isolator will let you run all kinds of stuff from electric blankets (have one on the driver seat now) to lanterns and charge jumper packs, etc... could get away for around 300 total for isolator/12 deep cycle/inverter.
With a buck converter like this
Amazon.com: 5A DC-DC Adjustable Buck Converter Reduced Voltage Regulator Power Module 4V - 38V to 1.25V - 36V (4 Pieces): Electronics you can power just about anything that takes batteries or DC input.