Steve, Fleming refrigeration systems are excellent systems. The addition of a 110 volt dual system was sometimes called a shore power system, unless the boat was equipped with a generator.
As to finding the leak in the 110 volt side I would insert fluorescent dye in with a refrigerant charge, then immediately remove the water hoses from the condenser and replace them with balloons to see if the leak is in the seawater condenser. After the condenser checks out reconnect hoses and run the system as much as you can to allow the dye to find the leak. Any small black light will make the leak visible if it is external. Leaks inside holding plates will cause severe distortion of a holding plate. Severe distortion means the plate will look like a football, I am assuming the plateís fill plug is still in place and tight.
Now to answer your questions:
To use another system connected to the holding plates it must match the capacity of the existing system. The key to your question will be the size of the tubing inside the plate. If the plate tubing is Ĺ inch OD a large 12 volt replacement is required. Plate tubing of ľ to 5/16 OD would allow a 12 volt Danfoss compressor like an Adler Barbour to be connected to a plate. On a boat with a standard DC power system there is a good chance with a large 12 volt system the engine must be run to support the large systems.
I donít know the size of your box or boxes, but if you have watched my slide show or read my 12 volt book you will be able to evaluate what compressor has the capacity to support the refrigerator. Your friends Norcold unit has either one or two Swing compressors in it, Swing compressor and others pictured in slide show.
Yes, there are ways to add a small 12 volt unit to the existing box without disturbing the holding plate systems. If you will send box sizes and plate sizes and locations I will give you some recommendations. It is best to e mail or use my refrigeration forum.
From the author of four books on boat refrigeration