First off, do you have one or two propane tanks? Did the automatic transfer regulator switch to the other tank when the first one went empty? Did you have the second tank turned on? After reconnecting, did you switch back to the newly filled tank and not run a stove burner to clear air from the lines? How fast did you open the tank valve? Too fast and sometimes the check valve will close it off.
Now, the little red LED for the 12V water heater switch. When you turn it ON, does the LED light up and then go off? Do you hear the burner ignite, or does the light come back on? If there's a problem with ignition, it will cycle through that three times and then the LED remains lit, indicating the system is in LOCK-OUT. Turn off the switch for 5 minutes and try again. You might go to the face of the water heater and try pressing the rubber reset buttons. Possible the 12V Hi-Limit switch tripped (for whatever reason) and needs to be reset.
You say you can hear it, but did you go out to the water heater, remove the cover, and look at the burner tube? If it's running, you'll see the flame, as well as hear it.
Do you have shore power available? If so, use the 120V heating system. Unless you have an Atwood water heater that only runs on propane, like my friend's SoB rig has (?).
