How much fuel should a reefer burn?

It might be running high head pressure which is the load on the engine.Go to gauges and check discharge pressure when it's on high speed cool,if she,s up over 300 psi try washing the condenser.You could also check the freon level when it's running on high speed cool below 32f,the ball should be floating in the sight glass above the battery.

 

I've been pulling reefers for about 16 months. In my experience, the worst I've seen is the reefer would last between 3 and 4 days on a tank of fuel (Reefer set to -10°, continuous and in Arizona in July.). At best, probably several weeks if it is set to cycles mode and the ambient temp is close to the set point.
In case you don't already know. The first and last quarter of a tank will burn really quickly. The gages aren't really set up correctly for a round tank. The top and bottom quarter of a tank physically hold a lot less than the 12.5 gallons that actually is 1/4 of a 50 gallon tank.
Being a company driver, I don't mess with them if they have problems. I know about refrigeration systems in general. Worked at a Air Conditioner place when I was a kid and have often taken care of my car AC systems. But, don't have any knowledge or experience specifically with reefer units on reefer trailers. I do prefer Thermo King over Carrier because the TKs are quieter.
Anyway, unsure if any of that was any help. But, though I'd share it in case it did help.

 

Have you tried setting the reefer on cycle/ start stop?

 

Trailer insulation will affect fuel consumption a lot.

 

My 2018 Great dane SS with Carrier 7900X4 eats about half a gallon per hour running continues. 24/7/365