Solar rules of thumb...
PV modules (solar panels) will on average put out 80% of their nameplate rating.
(They put out less when hot and more when cold. Since they are dark and pointed at the sun, they are usually hot.)
Flat mounted PV modules will on average get a decent harvest for about 4 hours per day.
(IF you park in full direct sunlight. Anything less than full direct (NO SHADE AT ALL) sunlight on the PV module and your harvest will be basically nothing. Even just a touch of shade on one corner can cut the module output by 1/3 or 1/2 off the top.)
So you can figure on average 80w x .8 x 4 = 256 watt*hours per day harvest.
Then, the charge controller (between the PV and the battery) will not be 100% efficient so you'll lose a few watts there, and the battery will only be about 80% efficient. Let's ignore the loss from the charge controller and just figure the battery. So 256wh x .8 = 204.8.
So you can figure on average that you'll end up with about 200 watt*hours per day actually getting into your battery and being available for use.
Not including recharging from the vehicle engine or a generator, your "solar only" power budget is 200wh/day.
Now you need to figure out how many watts your loads draw, and for how many hours per day and you can compare that to your power budget.
Say your fridge draws 30w when the compressor is running, and it runs at a 30% duty cycle (20 minutes out of every hour). That would be 30w x 8 hours of run time for a total of 240wh per day. That would exceed your "solar only" power budget.
An 80w panel isn't very big. If you rigged it with an extension cord and manually deployed it (leaned it on a nearby rock) facing the sun and moved it a couple of times a day, you could park in the shade and also get double or triple the harvest.
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