What the manufacturers normally install is a small solar panel to the chassis/starting battery to keep it charged while sitting outside and unused. Normally these are 5 to 15 watt solar panels and they just keep up if the sun is out, but just for the chassis battery. Batteries lose a small amount of charge while sitting. The solar panel and charging system on these are more like trickle chargers and cannot charge the house batteries or recharge a chassis battery that has been discharged. They can't be upgraded and often there is just a light on the dash board to show if the system is doing anything.

A solar charging system uses solar panels on the roof of the rig to gather sunlight and convert it into 12 volt power which is fed to and charges your house batteries. It is a passive system that works automically whenever the sun is shining. It will definitely allow you to boondock longer, but you will probaly also need a generator. You might take a look at Jack Mayers site, he has a lot of information there.
Good luck,

Fred (Luvglass) might be right that this is a more powerful and also more expensive option than I was talking about. But my experience is that the RV manufacturers do a lousy job of installing solar charging systems that will help you boondock, it is better to do the research yourself and either do the work yourself or hire someone who you trust. Here is Jack Mayer's info that Fred mentioned, http://www.jackdanmayer.com/rv_electrical_and_solar.htm . Expect to spend more than $2,000 and likely more to have something that will make a difference.

Thanks guys! These are on used RV's so I will have to check for more details but great information!