This is doable, but you have to watch out for a few things. In my scamp
, I switched out an icebox for this 12V/120V fridge: link
It is made for this environment, stays cold on the long drives (because of the 12V), draws minimal power, and is roughly the size of the other fridge (not exact though). I did have to modify my opening by 1" in each direction to get it to fit. It vents to the main compartment and has a fan to help the circulation. I ran it on a 90 degree day with the scamp
closed up and the fan didn't run and it stayed cool.
On my last trailer I just put in a 120V fridge where the icebox was. We only used it on trips up to 4 hours and it would stay "cold enough".
All these fridges need a little air flow. The link above designed it in. With the 120V fridge I had a big compartment I put it in and made sure air could flow above and below it. Your electrical
system has to handle the load which usually isn't a problem with most typical systems until you start running multiple things. I couldn't run the fridge, heater and frying pan at the same time on my last trailer. With off the shelf fridges we have had problems with the door opening as we travel so you have to make sure it doesn't happen to you! The fridge I linked to has a door switch to prevent that problem.
I hope that is helpful. Let me know if I can answer specific questions.