There are 'freezer monitors' available that can be set up to tell you what the temperature gets up to and for how long. Typically they are small battery powered units that either maintain a continuous log or start logging when a set temperature is reached. Most seem to have buzzers and/or small lights to give you quick status readings. The more complex ones will feed a detailed log to a computer.

Used to be these were high-end items that were prohibitively costly and used only in industrial plants and places like blood banks where temperature excursions were very costly or dangerous. Prices have come down, $25 to $75 roughly last time I checked, and the units have become much more user friendly.

My experience with an older, low-end refrigerator/freezer has been that in four hours the refrigerator compartment becomes a fetid swamp and the freezer is on the verge of becoming unfrozen. If I run the refrigerator thirty minutes every two hours the temperatures remain within acceptable limits.

My power plan is to hook a small generator to a set of large deep-cycle batteries running an inverter. Using all the excess power from the generator I can charge the batteries. With charged batteries and the inverter I can run smaller loads while the generator handles the larger loads like the refrigerator that can be run a shot time every so often. Shifting loads I can keep pretty much everything running that I need.

Should the generator fail or the loads prove too much I can run a second inverter off my vehicle. In effect turning my car into a generator to make up the difference or to limp along in a somewhat less satisfactory manner.