I received a VWRAS2-D12-D9-SIP isolated DC/DC power adapter from Digikey and built an updated voltage gauge module with dual gauges (one for the 12 volt accessory pack, and one for the 120 volt traction pack). The gauges were slightly too large to fit inside the dash, so I built an enclosure out of craft plywood that sticks out flush with the bottom of the radio.
One of my best decisions while building it was to use a tow light plug inside the dash that allows for easy removal of the gauge unit for updates, fixes, and general repair. This was very useful because the accessory voltage gauge did not work correctly at first. Unfortunately, the “isolated” power supply was not isolated enough for my 12 volt gauge. The first one gave inconsistent results and eventually fried. The second one I bought had the capability to use either a 5 volt power supply with a common ground or an isolated 9v supply. I tried it with the VWRAS2-D12-D9-SIP and it had the same problems, so I broke down and added a 5v linear regulator to power it using the shared ground mode. Problem solved. In the photo below you can see the two power regulators soldered up dead bug style and the back of the gauge holder.
One fun but unplanned feature of this system is that it has a “heads up” mode at night. The red LED traction pack voltage display is positioned so that it reflects off of the back window, directly into the field of view for the rear view mirror. So at night I can read the voltage gauge by quickly glancing at the rear view mirror.
Update: I have upgraded this system again with self powered digital panel meters.