Temp on empty trailer

Sure enough.

 

A trailer that old will burn more fuel and not cool (and stay as cold) as a newer one, but if it's all you can afford, go for it.

 

Changing temperatures is not a simple calculation. A reefer will cool from 100 to 60 degrees FAR faster than the same reefer will cool from 60 to 20. Going from 20 to -20 will take even longer.

Think of cooling a trailer to be like creating a vacuum.

A reefer does not put cold in, it takes heat out. The more heat you take out, the harder it is to take more heat out. Just like trying to create vacuum in a big apple cider jug with your lungs. You can pull a few mouthfuls of air out easy at first, but it won't be long before you simply cannot draw air out any longer.

There are lots of variables. Sunlight. Pavement heat. Ambient outdoor temperature and humidity. How well sealed and insulated the trailer is. The condition of the reefer unit.