In late 2003 Atwood changed the design/controls in their 2004 Model (and all models since)
Prior to that change....electric was wired directly for 120V AC and had it's own set of T-stats and ON/OFF switch. Propane was wired directly for 12V DC with it's own set of T-stats---circuit board and On/Off Switch
Newer Versions (4E for 10 gallon & 10E for 6 gallon)
Electric & Propane use same set of T-stats, same 12V DC circuit board for controls, 12V DC On/Off Switches on a common panel and electric uses a 12V DC Relay to trigger the 120V AC to element.
You would have to find a 2003/Previous vintage model
Update/change/add DC wiring to replace with NEW Model
See pages 23 thru 25 for info......
WHY do you need to replace current water heater?
Cracked tanks can be replaced/welded
Controls can be replaced
What is issue with current one?