That’s weird that a 3-phase generator was installed. Unless somehow they were cheaper than the 120/240v generators at the time. That’s when Monaco was having severe financial issues.
What are your old inverter/inverters do you have one, two or three of them? Also what is the transfer switch?
My guess is even if you have a 3-phase generator only two legs were hooked up to the transfer switch- that way there is no issues when you connect to shore 50a connection which is 120/240v.
Rather than replacing the 12v house batteries with a 48v-12v Dc-Dc converter, you may want to leave (a smaller) 12v battery in. Then when the slide or other heavy 12v draw items need power they can use both power from the converter and draw down the battery too.