This should be your number one concern.
You'll find pretty much all fridges these days will durable enough to go in the trailer (as long as it's strapped down firmly), but if your trailer seals air tight and you don't have a lot of open space around the fridge it will be working very hard and use a lot of power that it doesn't need to.
I have a 40L Waeco, my trailer is really well sealed and the fridge had a good open space around it originally. But I rearranged a few things and now it only has a couple of inches of 'free air' around it.
I noticed a huge change in my battery life after I rearranged the space around the fridge, so I did a few little experiments on my power consumption at home;
Starting with a fully charged 100ah battery (showing 13v) and the fridge sitting out in the open, it will run my fridge for 2 nights and 3 days and the battery still have 12.3v - 12.5v.
A fully charged battery with the fridge in the trailer and the lid left slightly open uses roughly the same over 2 nights and 3 days (~12.3v).
But, with the lid closed and the trailer parked out in the sun, after 1 night and 1 day the battery is flat (~12.0v).
It was really obvious to me that without the correct air flow around the fridge it runs constantly (creating more heat) and will eat your power supply.
During my test at home I opened the trailer after being sealed shut in the sun for about 4 hours and the heat that came out was impressive.
I had lots of people comment on how tight my fridge space was and now i know they were right.
Durability in a trailer, no problems.
Air flow around the fridge, critical.