connecting radio to 2 12v batteries in rv

A number of us have RV Motorhomes and usually the radio is fitted to main engine battery Which is good when engine is running BUT if we are parked up for long period and want to listen to Radio or CD fear running main battery down I there a way that our leisure battery could work in those times parked up saving drained main battery No battery drain down for either battery that the memory will reain on close down Could be a switch or blocking diodes in system Your help would be apreciated Bob

A normal dual battery system with battery isolator (automated) should be fine. Either run the radio off the 2nd battery else use the 2nd as a jumper battery (if it's suitable for cranking). A low voltage cutout (aka battery protector) could also be fitted to preserve the battery(s). For battery isolators, search for "UIBI" (charge light controlled relay) or if you don't have a charge (or not charging) light - ie, the D+ or L terminal from an alternator - get a voltage controlled battery isolator.