First off whether you have propane or not will have nothing to do with furnace fan not coming on.........
No 12V DC power to furnace WILL.
There is a small on/off switch---circuit breaker located in furnace compartment.
Need to remove access cover....check if it has tripped. Low battery voltage can cause high amp draw and trip it.
You can also check if t-stat is faulty.....remove cover and then connect the two blue wires together. Furnace fan should come on and fire off....if so t-stat is bad.
If not then.....got to get power to fan blower before any other testing can be done...such as 12V DC at the timed delay relay.
Here is a generic troubleshooting guide.......
LINKPost which model of Atwood furnace for more specific info.