I might add,the HOT ROD performs very poorly,takes a long time to heat up!!
Couple of things......
Hott Rod heat element is only 450W (6 gallon) and 675W (10 gallon) where OEM elements are 1440W--1500W so yes initial heating time is slower just using the Hott Rod.
Did PO routinely remove Hott Rod element and flush/clean tank and rod OR did PO install it and forget it so now tank is crudded up and element is coated????
Is this an Atwood Brand or Suburban Brand.
If Suburban Brand hopefully PO bought the Hott Rod kit with the incorporated anode rod----small that it may be but at least it has some corrosion protection.
As for swapping out for new combo unit.......
6 gallon combo fits where 6 gallon gas only (same for 10 gallon) provided you replace with same Brand (Atwood/Atwood----Suburban/Suburban)
New Atwood not only needs 110V power run for element it also needs 12V DC for turning AC element on.
12V DC (fused to ON/OFF switch to DC relay in junction box on back of tank where element is AND 110V AC (circuit breaker) to element.
The DC relay triggers the 110V AC to element.
Plus 12V DC wiring to connections for circuit board for electric.
Suburban......
110V AC (circuit breaker) to junction box on back of tank then to ON/OFF switch then to/thru 110V AC t-stats to element.
(Wiring from junction box to element is factory)
SO what is your pleasure?