I made myself a little 12v battery box for camping. Couple of 12v 15Ah mobility scooter batteries in an old plastic toolbox with a 12v socket, a few USBs, a couple of LEDs mounted on the side. Works great for charging things, running some lights in the tent, etc.
We have a little 12v compressor fridge (one of the chinese C20 ones), it’s fairly power efficient but it won’t run for more than about 24 hours on that setup. Considering doing a bigger ones but a chunky leisure battery means 25+kg to lug about to get enough to run it for a long weekend. May go down the route of putting one in our camping car instead and keep the fridge in there too.
As others say, hookup is nice if the site offers it but there’s loads of lovely sites that don’t.