For something inexpensive to operate, there are some compressor fridges that would cost about $30 a year to run. The compressor itself uses a lot of amps... but it has a short duty cycle, so it runs only a few minutes every hour. That way, one can have ice packs (stashed in a plastic bag) in the freezer part, and food/drink in the fridge part, also stashed in bags so it is easy to grab. When it is time to head out, open the fridge, grab all the bags (done right, it can be just one load), stash them in the RV fridge, turn that on, and head off. The ice packs will get the temperature down until the fridge can take over.
There are also always electric ice chests (Koolatrons), but they don't cool nearly as well, and probably take as much electricity overall as a compressor fridge (due to a 100% duty cycle.)