The Sprinter which an ERA is, has two 12V plugs on the dash. The cigarette outlet goes off when the engine is turned off. There is a another 12V plug under the dashboard center console that remains on all the time and another inside the glove compartment which I don't know the status because I don't use it and can't recall. They all run under the chassis battery and not the upfitter's house battery. I have a WeBoost cellphone signal booster under my dash which I assume is hardwired to the chassis battery (I should check) but then I also have a Trik-L-Start charger connected from my house battery to my chassis battery so keep the chassis battery charged from my 800ah lithium battery bank or indirectly shore power. If you are thinking about storage then you have to disconnect your chassis battery if you add loads to it especially if you don't have shore power and a means of keeping your battery charged, IMO.
2021 Advanced RV 144 custom Sprinter
2015 Advanced RV Extended body Sprinter
2011 Great West Van Legend Sprinter
2005 Pleasure-way Plateau TS Sprinter