I'm not sure if you want to power your fridge all the time or just keep the battery topped off when everything is turned off.
The basic answer is that you'll need a battery charge controller. It will keep your solar output from over-charging the battery. So then you need to size the solar panel to provide enough energy to keep the battery charged each day. Wanting to run the fridge all the time will be a totally different animal from just maintaining battery when it's not charged.
Once you know the energy needs, then you need to figure out how much sun you'll get and size your panel appropriately. This will depend where you park, where you live, etc.
In general - 100 watt solar panel is more than enough for my fridge in the summer. So a 100 watt solar panel would probably do what you need if you park in the sun. You could glue them to the roof for a permanent mount.
Or there are thin film adhesive solar setups that glue to the hood from cascadia 4x4 or lensun solar. They're peal and stick panels for the hood.
The reason most people don't use or want fixed solar on the roof is that where it's hot - you usually want to park in the shade and put your solar panel out in the sun.
If you only need a trickle charge - then a small 10w panel is probably plenty. I use one in my FJ40 to keep the battery charged. Just a 5w charger I stick in the windshield. Works great for that purpose.