Leveling: Some people complain about the need for leveling...but aside from the refrigerator's needs, I don't like living on an angle, so I always level the camper for me. I don't like it when eggs run over to one side of the skillet.
How level? I have a little bubble-level and I make sure the bubble is in the middle -- I carry several leveling planks for this purpose. I don't have an angle measurement. I think the owner's manual specifies a couple of degrees within level.
DC use while traveling: It will keep the food cold, but it doesn't have enough power to get a warm fridge cold...at least, not very fast. You need to have already chilled it down with propane mode. And if I stopped en-route for an extended time while using DC -- like to shop in a supermarket or eat a meal -- I always turned the setting OFF DC, either lit the propane or just turned the fridge off. I don't know if it was necessary to turn it off, but I know that DC draws a lot of power in a 3-way.
Eventually I discovered that it works fine to run it in propane mode while driving. It's not officially recommended...but many do it, and they don't blow up or burst into flames. Individual choice, obviously.
Overall I liked my 3-way fridge. It worked well and propane to feed it is cheap and convenient. They're used in the vast majority of RVs. I have a friend with a $50,000 Airstream trailer and it has a 3-way fridge.
Periodically (every couple years?) you may need to clean the propane burner of some kind of deposit/corrosion. I couldn't figure out why my fridge wasn't getting cold, but found (from a WTW thread) to remove and clean the burner with a wire brush: That was the problem!
The only thing I didn't like about my 3-way (Norcold, but very similar to Dometic) was when (after 5 years of use) the system ruptured catastrophically and spewed ammonia gas into the camper (and outside). I was driving at the time -- not in the camper -- so I didn't die.
My 2¢.
