How to achieve better power supply regulation for this remote scenario?
I might agree with Maple that it could be better to use a digital connection but if you use analog there are some things you must take into account:

In both cases (figure 1 and figure 2) you don't have a ratiometric output because you don't have access to one therm of the ratio , the power supply voltage at the sensor side. In the figure 2 it's obvious why , in the figure 1 due the unknown line voltage drop. 

I also don't see the reason to have an insulated DC-DC converter, what's wrong with a non insulated one in this configuration? In fact an insulated one would need to be a switched mode one that adds a lot of noise right in the proximity of the sensor.For the 3mA needed you can use a linear stabilizer.

From this point of view the solution in the figure 1 is better. 

To really have a ratiometric output you will need to have two more wires for signal ground and signal Vcc that don't have the supply currents passing through 

I would rather use this setup instead , at 3mA the voltage drop can be at most of tens of mV order, and do not alter the signal since the supply current goes through different wires. Also the noise from the DC-DC converter is far form the sensor side. 







simulate this circuit &ndash; Schematic created using CircuitLab

You can also put a linear stabilizer at the sensor side if needed