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What is the make and model of the water heater? That can be found on the data tag once you open up the outside inspection cover. Get back to me with this and I can help you sort this out. All follow ups are included in our fee.
If this is one for over a minute, then the issue is most likely in the thermostat or ECO assy. It is kicking off prematurely and not allowing the water to come up to temp. Are you camping now or at home?
Are you dry camping or at a campground? IF the ignition light won't come on then the thermal fuse may have blown down by the control board. Clear plastic with a resistor looking diode inside. Low LP pressure can cause the flame to "flash" and cook that. Try unplugging the leads on the control board and put them back on again. Same with the thermostat and ECO. It may be just a bad connection. See the attached link for a copy of the Atwood Service Manual and print it out. Lots of good information in there. http://bryantrv.com/docs2/docs/atwoodwaterheater.pdf
A 12 VDC test light or small digital multimeter is very helpful here. If you don't have those, get them when you can and keep them with you. About $25 for the both of them and easy to use. I can direct you in testing almost anything on this RV if you have those.
See the photo on page 18 of the front of the unit. The thermal fuse is on the brown lead. You can bypass that and see if that is the problem. I will make a phone chat offer for you as well. Sometimes it is easier to direct you in testing over the phone.
Just checking back in to see if you were able to resolve this issue or do you still need more input from me here?