2004 Atwood uses 12VDC for Electric and Gas operation/controls/functions
Same set of T-stat/ECO controls BOTH for heating cycles
This black box you removed....was that the cover over the element which is on the BACKSIDE of water heater tank?
A DC Relay with Yellow DC wire and then BLACK AC wires for 120VAC to element
White wire would be 120VAC Neutral
element is probably Good.
120VAC wires to/from element need replaced---or stripped to expose Good wire
Loose connections.....high resistance.......POOF/HOT and melted insulation
Just for reference THIS is the Frontside of Water Heater (outside compartment)