We run our aftermarket Xantrex Prosine 2.0 to run the fridge when driving. We also have 420 Watts of solar. The Trimetric shows what in going into or out of the batteries and almost always shows a small gain when running as above.
We do not leave it on all the time; During a cloudy day or night hours the fridge will draw 40 some amps from the battery if the engine is not running. When we stop for lunch or a rest we either switch to propane or turn the fridge off. We don't run with the propane valve open.