As shown in the picture, I will have 4 power supplies in series, three 48v, one 12v.

Can I power the volt gauge from the 12 volt power supply, and also measure the series voltage of all the power supplies?

Or should I use a separate power supply or an isolated DC-DC converter powered from the 12-volt power supply?

Thankyou for any help you can offer.

Below is a picture of the power supplies in series connected to the volt gauge.

Below are the connections to the volt meter/gauge.

 

If you have a multimeter, check the resistance between between the two negative connections. If you read a short-circuit then you can.

 

Good to go, as long as the 12 volt source has it's negative terminal connected to ground.