A refrigerator in an RV is generally an absorption type. They have no compressor, a heat source, usually LP gas flame and/or 120 v heating element provides the energy needed to create cooling. They often have small 12 v fans to help circulate air, but no freon or a compressor like a home refrigerator. The nice thing about an absorption fridge needs very little electricity to run (except the 120 v element) and has no pumps or other noisy components except the fans which are often like computer cooling fans.
The RV Doctor: RV Absorption Refrigeration Basics
A heating element or control board don't cost too much, but a new cooling system costs $600-$1000 plus installation.
If the refrigerator is a home type with a compressor, if the 'freon' leaks the whole compressor, condenser and evaporator must be replaced, it's a sealed system and can't be re-charged. It's often cheaper to buy a new refrigerator.
I suggest you find out the make/model of refrigerator in the RV, then use Google to search out the replacement cost. It will provide good information when negotiating a price.