first of macc you need to wire up your batteries into a split charge system, so that the leisure is charged up when the engine is running! next you have two options,
on a switch to opperate as you mentioned or permanantly wire the stereo in to the leisure battery!
peronally i would wire the stereo to the leisure battery, infact the easiest way of doingthis is to run a fused feed from your leisure batt to the existing fuse box, terminate this end with a male spade. find and remove the fuse that powers the radio, use a multi meter to find out which of the two holes the fuse slots in to is the 12v one, push the male spade into the other one! this way your lights, cig lighter and radio are all powered from the leisure battery.
this way when the engine is running it is chargeing the leisure batt and as such is effectively running the radio from the main battery/engine, and when you turn the engine off it you are already using the leisure batt for the radio, so no switch to forget about!
details of split chargeing on the wiki, top left, hammer icon! good luck