I'm guessing that if you are running it of your main battery (who's main job is to start your van), these are not very efficient and delivering steady low level draw over hours (unlike leisure batteries). I don't know but after about 20 amp-hours, you starting battery will be struggling to have enough left to start your van (particularly in winter as battery performance falls of a cliff when it gets colder + your van is harder to start: DOUBLE WHAMMY). You are only drawing at a 5volt rate, so assuming the built-in transformer is reasonably efficient, after about 40 hours of draw you will be running close.
Does the 512g have a low voltage protector cut off system; so that it switches off automatically before the voltage has dropped too low in your main starting battery?