The true "accessory" wire on a car radio usually is for a power antenna on a car, or an external amplifier.
Now, if you have a two input radio, meaning there are two hot wires going to the radio head, one will keep the electronics going (keeping stations pre set etc) and the other will power the radio proper. Your wire for "accessory" you mention may mean it goes to the Accessory position on the car, that is switched on and off with the ignition.
So, not sure what to advise here, is it an accessory to the radio, like a power antenna or does it mean feed from the accessory position on the car. You may need to read the instructions with care to know which one you have.
When putting a car radio in an RV, it is easy to wire it in a way it will always be draining the battery when not in use. I always put a positive on/off switch in series with all power feed lines to the radio. You may lose the station pre set and clock when it is turned off, but your RV battery will not be killed over time. That is a good trade off in my opinion.