Your SW6DEL uses 12VDC for control functions ---electric and propane
Both electric & propane om/off switch have 12VDC supplied to them from a FUSED DC Circuit.
Electric side uses a DC/AC Relay to trigger the 120VAC to element.....similar to Atwood combo unit
BUT the DEL still maintains separate sets of t-stats under the 'Push To Reset' Cover
120VAC/electric....Left side ---- normal t-stat/opens 130*F & manual rest High temp/opens 170*F
12VDC/propane......Right side --- normal t-stat/opens 130*F & manual rest High temp/opens 170*F
Inside the DC Junction Box (left side of water heater tank---from backside) is the DC/AC Relay
CHECK the AC wire connections on that relay
Changing out the element after 8 yrs is not unreasonable
