I have a 48V battery bank and a 12V house. Been running this setup for 4 years now and no issues.
I use an MPPT (Victron SmartSolar 100/20). Connect your 48V battery to the MPPT PV input. The MPPT sees your 48V or 52V bank as solar and it outputs proper charging voltage for your battery type. You can set charge voltages, and all the charging parameters. MPPT's are designed to do exactly that, so much better than a converter. Chances are you already have a Victron unit for your solar arrays, if so you can use the same Victron Connect app to read the house battery unit as well.
Additional tip. I charge the 48V bank at the dock (shore power) as well as solar when cruising (using another MPPT). To prevent our 12V loads discharging our 48V bank when at sea or dock and end up without a motor, we use a switch to the victron unit remote connector. This is not very well documented or understood, but the VE. Direct connector can be used to turn the unit on or off without requiring the VE.Direct cable or unit. Connect Pin 2 to GND will switch the MPPT charger off. Switch that connection to open and the unit starts charging.
Keep in mind that the 48V whether connected through a DC-DC or MPPT, the 12V battery is being maintained all the time from your 48V hence draining your 48V bank, unless you have a 12V disconnect or a charger disconnect.
In my case, at the dock, a relay opens when we have shore power, and allows the MPPT to charge the house battery. When shore power is removed (when at sea), the relay closes and the MPPT for our 2V house is off. We use minimal 12v loads and monitor the house battery level. I also installed a bypass switch that I can manually through open if needed (after confirming sufficient reserve in the 48V bank). So at sea, if the 12V is starting to get low, we flip the switch and the MPPT starts charging our house battery from our 48V. Very nice and peace of mind, no worries about the house loads draining our 48V bank while sleeping, anchor light staying on, etc..
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