Hi CeeCee,
There have been a ton of threads on this problem over the past several years. A simple search on the forums will yield hours worth of reading. In fact, if you scroll down to the bottom of this page, the website provides likely matches as "similar threads." That owner history and ingenuity is worth a ton, impossible to replicate here, and will include pictures, sources, etc.
There could be some real issue with your door seals or something going on like that, but there are a few inherent problems with refrigerators installed in slides that match your symptoms. The single biggest contributor is a lack of airflow (heat buildup) behind the rear of the refrigerator. The heat removed from the box is dissipated there and has a hard time getting out of the upper vent on the side of your rig on its own.
The net result is that the refrigeration system runs out of capacity and since the freezer gets the lions share of cooling it appears fine. I'll also bet it is not near zero degrees where it should be.
Many of us have installed additional condenser fans, and airflow baffles to help get the heat out. Another helpful 'mod' is to add insulation where possible between the top and side of the refrigerator and the slide ceiling. A small fan (available from many sources and ebay) inside the refrigerator by the fins also helps to even the interior temperature. These 'mods' do a lot to help.
I'd encourage you to do some searching and reading so that you can determine the cause of your particular problem and solutions that you might implement.