If it's a separate deck mount converter, it could be most anywhere, not necessarily very near the electrical panel. Attempting to trace the wires from the house batteries or the DC distribution panel may be helpful; the converter would usually connect to one or the other of these components more or less directly. There are many variations in the exact wiring topology, though.
Some converters (particularly the older ferroresonant ones) you can locate by tracking down their sound. Some are more or less noiseless, which is great all the time except when you need to find it.