Here in Argentina dedicated switching units like the Red Arc line of products are not available. Very few dedicated 12V fridges are available and extremely expensive. I elected to use a small house-type fridge that runs off 220V. The voltage may be only 110V where you live. My system is designed to perform all the switching functions using discreet relays for 12V sources, and contactors (same as relays) for 220V sources.
When the truck engine is OFF and the truck is not hooked up to shore power, the system draws power from the home/leisure battery or solar if enough sun energy is available. The inverter converts 12V to 220V for the fridge and LED lights. When the engine is turned ON and the driving lights are switched ON then a couple of 12V relays switch the source from the leisure battery to the alternator. The inverter functions exactly as above.
If the truck is plugged to shore power (campground 220V) then the 220V contactors switch the 220V lines from the inverter to the shore power.
Note that here we cannot get battery-to-battery chargers either. I brought mine from Canada. If you cannot get a battery-to-battery charger where you are you might get away with a smart relay that will link the two battery only when the engine is running. This will not work well with vehicles that have a smart alternator (most modern vehicles). In that case watch my video
Below are the schematic diagrams. You are welcome to download, print, and use them for your personal use. There is also a link to convenient and free calculations spreadsheets.
Please note that all the technical information is provided in good faith and as an educational resource to help the reader understand how such a system works, but it is no replacement for professional advice if you intend to build such a system. The system I describe here works on my vehicle with the equipment I use but you might get different results with different components and/or a different vehicle. Do not simply duplicate my system unless you fully understand what you are doing. Never take chances with electricity.