I was a bit slapdash with my reply, I'll admit I was on my phone at the time.
I certainly agree that RXP plugin definitely should have more customisation options after all this time. The market for add-ons has grown far beyond purely default systems/plane maker after all. May be "fakable" by using one of the unused buses?
If RXP plugin doesn't save this option to the INI when selected via the UI that's definitely a bug, but also not terribly surprising, tbh...
I had to go look it up when I got to my PC to confirm (not in the documentation because that is also shockingly bad, but examples from some X-Plane 11 add-ons), but this should work as part of the gauge settings, or some variation there of (may require some trial and error to figure out based on the manual quoted above):
; sets device power source: ALWAYS_ON, AVIONICS_BUS, MAIN_BATTERY, etc.
PowerSource = AVIONICS_BUS
Cheers.