Non-technical answer...
You
should be ok if you`re using the vehicle`s main battery to power your Engel for a day trip or for an overnighter..
But IMO, if you`re gonna use your new fridge on a frequent basis, or you intend to "stay put" in a camp for any period of time (more than a day or so) ya might want to do yourself a favor and install a secondary power source now.
Keeping a constant, watchful eye on your battery`s condition will get you by on a short trip, but the LAST thing that I`d want to worry about while enjoying the great outdoors is the discharge rate of my
only battery.
Running the engine in the middle of the night at the campground because your voltage is showing nervously low will not make you the most popular person in camp either
If you don`t want to add a full-blown dual-battery system to your rig right away, just add a deep-cycle second battery that you can charge each time before leaving.. wire and fuse it to the rear .. then use it exclusively for the fridge and/or accessories back there ( Just be sure to include a way to "tap" into the main battery if needed ) you should be able to run your fridge for several days on end with the second battery.
You have Slee`s battery tray ? ... Heck, you`re already half way there !
Regardless, I would do as many "tests" with your new fridge under various conditions ( empty, full, + or - ambient temps, use it as a fridge and as a freezer etc.. ) to get a fuller understanding of its true energy requirements.