It is 8F or -14C in my battery compartment right now. 
 The camper design is not convenient to do a total rework of battery location to inside the heated living space and wiring. I’m scouting for ideas. 
Is anyone using an external heating system on their battery bank? 

ths guy just showed his setup for warming the batteries

He used car seat heating pads. They would work poifect for what I want to do. 

Thank you for the link Ed

ebay the seat heaters are $17 for bottom and back. I know because I just ordered a set for my buggie

Holding tank heaters are also a possibility.

Click For Full-Size Image.

This little heater worked well for our two weeks below -20f and kept the compartment a toasty +40f. 

Edited November 30 by DesertMiner

why sometimes Heater not working in my saturn lon? could anyone please share with me your experience.