You might be able to get away with a galley side slideout in.
Tembo Tusk makes them. Option 2, and probably the cheapest is tongue box. Just get a top opener large enough to give you access to controls. But my own personal preference if you're only running one fridge/freezer has always been in the tow vehicle. Even when I'm not camping or exploring, it's just nice to have to do simple things like keep groceries frozen and not have to worry about running home right away. Or maybe you're going to a sporting event and you pop open the tailgate and have instant refreshments. Or you make camp, detach the trailer and drive trails all day, you don't need to worry about bringing a cooler. I've also used it for storage of pretty odd things. Once I bought a floral arrangement and didn't have anyone to hold it and keep it from spilling. So popped open the fridge, vase and all down in there (the ARB has a removable lid) and what little water did spill was all contained by the fridge. I don't know your setup, but with a 4Runner, I'd put a
retractable, tilting slider by AluCab in there and be done. It's pricey but it's sturdy and awesome. (Why is overlanding SO expensive?!?!)