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That would be either a thermostat or a timed delay relay issue. IF you hot wire it and go around the thermostat and it comes on, then you know that is the problem. IF it won't come on that way then it would be the timed delay relay. Small black "cube" with the thermostat lead and hot lead and the positive lead to the motor coming off of it. About $40 or so and most RV shops would have one. Hope that proves helpful here. Please get back to me if you need more and I can supply a manual and we can dig deeper into the problem. All follow ups are included in our fee.
I am attaching a link to the Atwood Hydroflame manual which covers this model. Print out a copy so you have it. http://bryantrv.com/docs2/docs/hydroflameold2.pdf
Just checking back in to see if you were able to get this sorted out or if you need some additional input and assistance from me here.