Check for restriction in air return AND air vents/ducts.
Check voltage at the furnace. As indicated 12.0 is WAY discharged. As a test, put a battery charger on so that voltage is above 12.5 and see if that changes things.
If it does, check voltage at the battery and again at the furnace with the fan running. If more than a .2 VDC drop in the wiring, up-size wiring, clean connections, etc.
Do NOT start by assuming furnace internal problem. The SAIL SWITCH HAS TO CLOSE FOR THE FURNACE TO TURN ON THE PROPANE AND IGNITOR. Insufficient air flow for any/a combination of reasons will keep this from happening.