We've had this 12v compressor fridge installed in our RV (self installed, self built motorhome) for 5 years. When it was first installed it only needed 12.75v to start it. Then, after one year it needed about 13v, another year 13.5v.... Now it needs 14.25v to start, but our alternator won't produce that. When we are plugged in to 220 (we are in Europe) the battery charger will produce just enough and the fridge will run fine. 

What is our problem, and how do we fix it?