What model of Atwood?
You talk about a pilot, automatic lighting, talk about a circuit board and WH being a new one :H
Circuit board, auto light so WH is probably a DSI (Direct Spark Ignition) that uses a spark electrode to ignite Main Flame.
As suggested by other posters.......check the thermal cut out (thermal fuse in a clear tube connected to brown wires on T-stat.
IT trips if temp inside outside compartment goes to 190*F or higher (usually due to a blowback from combustion chamber....dirty chamber/burner tube/obstructed gas flow) and cuts DC power.
Thermal cut out is a one shot safety device..if blown has to be replaced.
You can remove it and test function of WH.
Disconnect it from circuit board and t-stat....connect wire from circuit board to t-stat and then turn WH ON.
Should spark and main flame light off.
You can run it like that until you get a new thermal fuse.........but make sure main flame is strong/steady/and not coming back out of chamber.
Clean chamber/exhaust 'U' tube and burner tube main flame is erratic
