I use a Buck/Boost Module for my Camper/Motorhome.
They are available for a reasonable price (think insurance) on various auction sites. You need to make sure it is capable of supplying adequate current for the Router. This description should lead to a 3A version. I do protect the supply and output side with a fuse. I have run my newer GL-X750V2 and previously, a DWR921 from this.
3.5-28V to 1.25-26V DC-DC Converter Boost Buck Step Up Step Down Voltage Module
Disclaimer: This is just what I use, so far, I have not had a problem. But…
I would strongly recommend looking into some filtering/protection if the router will be connected directly to automotive power at any time. Automotive power is notoriously spiky & noisy.
I don’t know specifics because I haven’t gotten this far in my camper build yet, but there are definitely components out there which can be used to filter automotive power and prevent noise from causing data corruption, or voltage spikes from causing a dead router.