Welcome to the complexities of electricery ewokstar!!
The first thing you need to do is check your battery capacities. Cracka has given the example that he has two X 100Amp Hour batteries, giving 200Ah total capacity. He supplements this by solar charging, which mite put back in somewhere between 20 and 30 Ah per day, depending on panel capacity. This system gives him the 5 days use period you require.
You also need to consider how the fridge is going to be used. If you sit it in the sun, keep opening and closing it, and keep putting extra
into it to cool it down, it will have to work a lot of the time at the 4 amps per hour use rate quoted. But, if you treat it nicely, keep it in the shade and talk to it often, It will use less! At the 4Ah consumption rate, it would chew up a lot of a normal 100Ah battery capacity on a hot day (4 amps x 8 hours equals 32 amps used).
Yes, engels are pretty efficient. The average consumption rate over a day/night may be something like an average of 1.5 Amps per hour (may be a bit more). This means you will use approx 30 Amps per day. Simplistically, If you have a 100 Ah battery, after 2 days use at this rate, you will only have 40Amps left (40% charged). If its a dry cell battery, it wont like going repeatedly below this level, it will shorten its life. If its an expensive AGM battery, it will tolerate this better.
So, in the first instance, its a few basic sums on battery capacity in Ah, and then look at your potential current draws. I would say however, that if you want to be sure you can run this fridge for 5 days whilst away, and if you only have one aux battery in the camper (of say 100 or 120Ah), you will need some form of auxillary charging. Remember, you are also going to be running lights, etc, all of which use a few amps per hour when in actual use when totalled up.
