If your water heater dies, here's a quick way to find out if it is the Thermal Cutoff. First, MAKE SURE there are no obstructions in the flue tube. Then turn off the water heater's electric mode and turn on the gas mode. With the exterior access door open and looking at center of the water tank you'll see a black foam-covered area with 4 wires running to it. Look for the area marked T-STAT with two brown wires running to it. One of the wires will have a piece of clear tubing over it that covers a large diode- this diode is the Thermal Cutoff. (Mine didn't look burned or in any way defective, so a visual inspection won't tell you if it's bad or not.)
Remove the diode from the T-STAT terminal lug then disconnect the other end of the diode from the brown wire connector. Now connect the brown wire (without the diode attached) directly to the T-STAT terminal lug. Give it a few seconds and the water heater should turn on in gas operation mode. If it does, that means the Thermal Cutoff is bad and you'll need to get a new one from Atwood. Leave the wire hooked up, turn on the electrical mode to supplement gas mode (if you want) and you'll have hot water till you get the new Thermal Cutoff.
If the water heater doesn't turn on after a minute or so chances are it's not the Thermal Cutoff and you'll have to look elsewhere for your problem.