VERIFY the clearances (actually lack thereof in most dimensions) of your coach's refrigerator installation.
Open areas above the refrigerator and excessive back of refrigerator to outside wall dimension really cut into refrigerator performance.
And if in a slide out (already diminishes cooling performance) a "transition" from top of refrigerator to the top grill helps.
Is performance the same on both 120 VAC and propane? If one is inferior to the other, troubleshoot the one not performing up to specs.
If all that meets the installation specs that came with your refrigerator, a partially plugged cooling unit is a likely suspect.