Ambient outside would effect it. Inside may have a little if the reefer's door seals are not tight.
Use a dollar bill closed in the door to test. It should take some resistance on the bill to pull it out. Do it top, bottom and couple places on the side.
Open the outside access to the refer. Check clearance between the outside wall and the cooling pipes. There should be less then an inch of clearance.
If you have more you could use a piece of foam board just above the access to close it off. I used a piece about two inches wide, length of the refer opening and mine required about 1.5 inches thick. Attached with Liquid nails. This should make more air go through the cooling unit to make it more efficient.