Are you sure the electric element is heating?
Suburban uses a 12,000 btu/hr burner (2000 btu larger than Atwood)
With gas/electric it has the fastest recovery time (10 gallons/hr with gas and an addition 6 gallon/hr with electric)
Either the electric is NOT working
OR
Cold water is mixing with Hot.......bypass valve leaking thru/not closed or outside shower not turned off with hot/cold knobs (just on handle)
Before replacing unit I would verify that both sources are heating.....and cold water isn't being mixed with hot
Besides....those on demand leave a lot to be desired in RV use
