NOCO GENIUS10: 10A 6V/12V Smart Battery Charger – Automatic Maintainer, Trickle Charger & Desulfator with Overcharge Protection & Temperature Compensation – for Lead-Acid & Lithium Batteries

Cheapest/easiest way would be to run an extension cord from your lithium setup to the vehicle battery and plug in a battery charger like this:https://www.amazon.com/NOCO-GENIUS10-Fully-Automatic-Temperature-Compensation/dp/B07W3QT226 But honestly, why not just buy a 12v fan, buy a portable radio, get some 12v lights and install them in the cargo area of the van and run them off your battery system? Maybe get some headphones so you don't disturb other people around you and so no one knows you are in the van.