For those of you who only wish to maintain a bank between trips, simply add a good charge controller that has a battery temperature probe on the battery bank (my current favorite is from Grape and is capable of up to 40 amps of current on the output side) and 15 watts of panels per 100 amp-hours of storage.
How do I know 15 watts is enough? Because on my first RV I had just 30 watts of panels to maintain 250 amp-hours of battery bank. Between trips that was enough to return the battery bank to 100% state of charge if the RV was in storage for two or three weeks.
For the rest of us, an energy audit needs must be done to design a system that will meet our needs.