Steve, I'm going to have Fred install a Samsung 18 when we get home as a replacement for our Dometic. I was told the current draw is 1.9 Amps AC so I had figured ~ 15 Amp DC at 12 VDC give or take. I also figured a 60% duty cycle which would put me at only an extra 9 AH of discharge....not very significant. I suspect if one wanted to really conserve juice the freezer temp could be raised to....say 15 or 20 and the the fridge to 40 ish and shut off the ice maker (optional) thereby extending the duty cycle. Your Dometic probably doesn't even run that cold most of the time.
One other thing I might add is to measure carefully before you make the size decision. The 20 is exactly the same size except the depth is 2" deeper. When I spoke with Fred about the installation he said he needs 1 1/2" extra for access to the mounting brace and for the relocated electrical outlet. With that info a 20 might fit in mine but there is a chance that the fridge may protrude a bit further into the living area and I want it recessed far enough so the cabinet frame is even with the door gasket. Also, while the 20 might be nice in reality the 18 is so much larger than out 12 cu ft Dometic it's not worth potentially compromising the aesthetics. I'm probably going to go with the black vs. SS as I think it will go better with our interior and existing black appliances.
