and the other solution (daytime only) is a couple of solar panels.
The best of the DC->DC boosters is this one, which can be monitored and programmed from a cellphone. I recommend this one - but be sure to get the "Smart" version, not the cheaper dumb one without cellphone control. If you go over 15A, your bargman cable might be at risk of damage due to overheating (I burned out my own original cable by doing that), but just 12a on the input side should support most of your fridge DC power requirement. That might be around 120 watts, maybe a bit more - and the fridge uses around 140-150 watts in your high-voltage situation.
https://www.amazon.com/Orion-Tr-12-3.../dp/B0851TPKV7
A couple of 100w solar panels (peaking at 200w at "noon" under perfect condition, you'll actually get less than that) would work for daytime, but not early AM or evening travel. They would possibly also help to re-charge the batteries in camp.