You might need an RF choke in the power cables to your radios. Also, check your ground. Not just the DC converter but also the radio. DC/DC converters, as well as inverters and switching power supplies, need to be in a metal case and the case MUST be grounded. You might try running both + and - wires several turns through an old salvaged toroid just to see what happens, too. Maybe thread a few ferrite beads onto the power supply to your radio. Another trick is to connect a capacitor between + and ground with a resistor or inductor in series.
If ARRL still publishes their handbook, you might want a copy. It is full of useful information even for the non-Ham.
Do you have an AM broadcast receiver? Tune up and down and see if you detect noise at equally spaced frequencies. That would be harmonics of the operating frequency of the converter, which is actually probably a switching mode power supply. Essentially a very powerful oscillator running at somewhere between 10khz and 50khz, I am guessing. This low frequency RF is leaking out of the dc converter and radiating from the output and also the input wiring. At least, that is my guess. So if you hear that equally spaced interference on your AM radio with the converter on and it goes away with it off, that narrows it down a lot. Oh, and be sure to turn off ALL other loads. It could be RF generated elsewhere.
Sounds like your house bank is ready for an upgrade, to me. You should be able to run your house stuff and comms all day and night, at the very least. You might also want to upgrade your lighting, both house and nav, to LED. What type of batteries are you using for your house bank?
All power supply wiring should be ran in pairs. You don't want to have + and - separated. Twisting them together is even better. You want fields to cancel out as much as possible. Don't count on your boat's ground as a return line. (that also can cause electrolysis problems.)
Small 48v/12v converters are crazy cheap if you can get by with just 10a or so. And you can always run more than one, too. A dedicated one for your radio and one for your lighting, for instance. I have ran cheap dc/dc converters without the problems you are experiencing, so there is hope, yeah.
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