Hello,
My son and I we own an old firefighter Mercedes car and we are building a camper vehicle in its cabine. The voltage on board is 12V and we will have USB A and C sockets at the walls.
Is it save to charge the L5 directly with 12V?
Thanks
I would say “no”. Not safe and might not work even if it doesn’t destroy the phone.
In my car I use a cigarette-lighter-slot adapter that gives a (hopefully) compliant USB-C PD power source and that works fine with the Librem 5. The difference is though that this is an off-the-shelf car. I am not building it myself. 
That said, I have no idea what voltage the cigarette-lighter-slot provides.
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Technically, the Librem 5 is able to handle up to 14V on VBUS.
In practice, it’s a very bad idea to expose 12V on a USB-C pin (and even worse on USB-A) without performing PD negotiation, and the L5’s PD controller config is set to never negotiate more than 5V because of the risk to the 5V-rated CC lines (which are next to VBUS in USB-C) in case of a short. Older batches were very susceptible to damage from that; on Evergreen the risk is small, but AFAIK still non-zero (say thanks to TI).
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So far as I know, all “new” (1970+?) vehicles use 12v, negative ground. My cousin had a 60’s Austin that was positive ground (positive “earth” UK).
My old military trailer was wired for 6v, so that old fire wagon could be anything. Check the battery!
Yes, I think you are right. So if the OP wants to wire in a car charger like mine (which offers a USB-C PD port for the Librem 5 (and for more recent iPhones) and a USB-A port (for older iPhones), maybe it would work. So the car charger handles the conversion from 12V to whatever is the negotiated phone charging voltage. However that might not apply in the EU and it might not apply to the vintage of car that the OP is working with.