Sounds like either the control card or the pilot light sensor is not working as it is supposed to. First, determine the model number of your heater as you already stated that it is an Atwood. If the electrical schematic is still legible in the heater compartment, look at it (and yes, a magnifying glass is helpful) to determine the model number of the heater. Look at the control board, there should be a LED on it that lights up and flashes when the heater finally faults out. On the schematic, there should be a truth table that translates the flashes to the fault the card is having. Light the unit as normal and allow it to fault. As an example, if the LED flashes like this: On-off-on-off for three to five seconds and cycle repeats, you are looking at a 2 cycle fault code. The truth table would tell you what it means. Some of the faults can be adjusted by the user while others need a replacement items or pressure gages that you will not likely have. Contact the service department with your information and they will be able to tell you a ball park of time, parts and costs if the repair is not in your range of skills.