I have:
The windless battery is charged off a 120v/12v charger and only charges at the dock or from the generator. I would like to charge both the engine and windless batteries from the house bank. I assume I could do this with dc/dc chargers one for engine and the other for the windless. - Ken
You are correct that the DC-DC chargers are the correct way to go for this purpose, if all your banks are 12vDC the best option is for the Victron Orion 12/12/30. Here is a link to one on our website, note it is the non-isolated model so all of the battery negatives need to be tied together.
https://www.pysystems.ca/products/victron-orion-tr-smart-12-12-30a-dc-dc-charger-non-isolated/