I am looking at DC fridge models also. I have a Coleman DC cooler, $100 or so. It runs all the time, so will make things very cold on a cold day, or just cool on a hot day (ambient temp).
I run off of two Golf Cart type batteries that are 6 volt run in series to make 12V.
It has about a 150 watt solar panel also.
On a cloudy day with full batteries and only running the cooler (maybe a little water pump) it will go for 24 hours easily, but is loosing voltage and the cooler is slowing. Seems that 2 days on the typical two-battery set-up is about average from what I am seeing. This is what people want to know when they are looking for cooler/refrigerators. How long will it run, is the big question.
With the supplemental solar panel on sunny days I can run 3 days, and if I had another panel I bet I could run indefinitely. The one panel I have seems fine in the sun, but it is at night where it loses the battle and cannot recharge batteries fully and run the cooler at the same time. One step forward, two steps back with the charge - so to say…
My little Coleman cooler is doing the job, but temps have been cool at night mostly, but additional insulation would help.
The cooler can run top open, or upright with side open. I travel with the door on top, and when set up in camper have the door open like your house fridge as I put it on a shelf to save space vs on the floor or table with door on the top.
I do not know about running it any other angle, but I like it over propane only in that I will not ruin it running it out of level. Propane units are so needy when it comes to being level when stationary. By out of level, I mean any tolerable angel that I might park at. I would never park at an extreme angle for camping, like the 30 degrees people mention.
Amp hours. Take the amps the unit uses and divide by the available amp hours of your battery, then divide by 2. Divide by two as you really can't run a battery to zero and have the accessory work right, not like you can when you run a propane tank to zero. When battery gets low there is not enough juice to run your accessory, and running battery past half charged is not a good thing for the battery.
So, maybe a 100 amp hour battery will give you 50 hours for a 1 amp draw, or 25 for a 2 amp draw. Also, this is assuming it runs all the time. For units that only run 50% of the time you can double the time until battery depletion.
So, example, a 5 amp draw on a 100 amp battery would yield 20 hours, but if it runs 10% of the time then you would get 200 hours - then divide by two for 100 hours/ 4 days of runtime before the battery is not providing enough power to run it correctly. I think I did that math right…
Also, batteries will lose voltage on their own over time.
Unless you are just a weekender then supplementing with solar or generator is necessary.