Generally, the more level the better, but the object is to keep the boiler area of the cooling unit fed with enough ammonia solution. I was recently introduced to a new safety item which eliminates the danger of off level operation by sensing the temperature of the boiler, and through a reasonably sophisticated algorithm, shutting the power off (and restoring it). The original webite for this product is 
here, with a website that I did for them- gratis, because I believe in the product so much- at 
http://healthyrvrefrigerator.com/ (still a work in progress).