Just as a point of reference, my dash radio is connected to the house battery (only). The connections vary from builder to builder and maybe even from motorhome to motorhome. It's pretty easy to figure out how it's connected if you have a house battery disconnect switch: if the radio stops working with the house battery disconnected (and the converter not operating), then it's obviously connected to the house battery.
If it's connected to the chassis battery, you could just get and install a Trik-L-Start or Amp-L-Start and be all set. I very much doubt that the typical standard dash radio would draw over 5A average in normal use, even though it may do so momentarily.