Heating pads such as the # 277-000164 are designed for 29 to 40 gallon tanks which are much larger than the tanks on your Lance 650 truck camper. My fear is that they could possibly get too hot and damage the tank.
From what you describe, a solution such as a self-regulating heating cable such as the Valterra # W01-1573 for a 25 foot or # W01-1574 for a 50 foot length would be your best bet. The cables operate on 120V AC power, so you'll need an inverter like # DU89FR to power them.
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