Commercial auxiliary heaters for medium / heavy trucks are simple box heaters with a blower (Red Dot is one manufacturer)
Old Willys pickups had very simple box heaters also ( when heating the cab was an after thought)
A couple of examples:
https://www.amazon.com/Auxiliary-Hea.../dp/B00E5ND62C
https://shop.willysamerica.com/Box-Heater-p/94534b.htm