YES the '3E' version was a combo gas/electric. The electric element is a direct wire design......AC CB to ON/OFF switch to/thru set of t-stat (located on back of tank) to element.
Propane was DC with it's own set of t-stat
In 2004 Atwood changed designs.
Electric and propane use DC for control via circuit board and a relay to activate the AC to electric element. This also allowed for just one set of t-stats for both electric/propane.
Change can be done just need to use DC on/off switches, run some DC wiring for electric on/off---relay.
Still use the AC CB and wiring..just needs to be triggered by the DC relay