The best place is in the fridge because the fridge needs to be level to operate efficiently. Once you have the trailer leveled so the fridge is level, check other places to see is they also show level. If you're lucky, the floor right inside the door will read the same as the fridge so you can just open the door and check there.
The other thing you can do is level the trailer so the fridge is level then install a level like THIS
on the outside of the trailer where you can see it from the tow vehicle or levels like THESE
in the vicinity of the front jack.