From what I have read, the more level the TT is the better and more efficient the fridge will run.
Before we go out I hook up the TT to the TV and pull around to the front of the house for at least 24 hours. Our roads are somewhat level and I try to make the front/back somewhat level as well (while still hooked up to the TV) but I can bet I am at least a bubble off of level either way. Our fridge / freezer cools down perfectly..
The following excerpt is from. http://bryantrv.com/reefer.html
While most modern RV refrigerators will adequately somewhat off level (the popular recommendation is that if it is comfortable to walk in your rig, it is good enough for the refrigerator), they will still work better and last longer the more level you make them.