You might want to consider an "Echo Charger". It is designed exactly for your application, and is a very easy install in many motorhomes. Installation is only 3 wires, one on the starting battery bank, one on the house battery bank and a ground. When the house bank gets above a preset voltage the echo charger allows some current, up to 15 amps max, to charge the starting bank. Both banks would stay charged as long as you are plugged into shore power and your house battery charger system is working properly. If your motorhome has a solenoid, battery isolator, or some other location where cables from both battery banks are fairly close to each other, that is somewhat protected from the weather, the Echo Charger can be mounted there and just connect one wire to each bank.
Hope that link works, if not just do a search for Echo Charger; lots of places selling them.