Your coach **probably** has a "Converter/Charger" for your 12VDC needs. That's Interior Lighting, Refrigerator, Water Heater and Furnace. The type of C/C I'm familiar with:
1. Powers the above and more with 12VDC House Battery power when there is NO 120VAC available.
2. When 120VAC IS available, the C/C takes on the above, relieving the House Battery of those loads. Therefore, Furnace should run and be able to run indefinitely, not limited by state of House Battery Charge.
3. When 120VAC IS available, the C/C ALSO recharges your House Battery.
Sounds to me like your C/C is not working. Either not connected, breaker or other fuses/switches off, or defective.
New Coach? Do you have the documentation that came with it? Should be info on the C/C. Post its Make/Model here and somebody can provide more detailed help.
New Member? Welcome Aboard and stay with us for a Happy 2015 and Beyond!