Most of those original and somewhat experimental (by Airstream) systems are long gone and forgotten, and it is just as well. As I recall, some of them had a 120 to 12 volt
transformer and a relay. If the unit was plugged in, the 12 volt
system got power from the transformer, and it may not have even been DC. But it might have had a diode or selenium rectifier to make it into DC, I am not sure at all now.
There is just no reason with today's systems to try to replicate the original, unless you are doing some museum quality piece.
It is easy to make a simple 12 volt DC electrical system for your rig. Get a Progressive Dynamics 9245 converter/charger and a 12 volt deep cycle battery. Charge the battery with the PD, and add a simple fuse box or individual fuses for the 12 volt circuits you will use, such as the water pump and lights. Add a charge line from the tow vehicle if you want, be sure to fuse it at the trailer battery end. You can use that wire to run a power jack if you want one. A few light circuits, pump, maybe a furnace and you are up to date, but simple.