BATTERIESNOTE: This is not meant to be an exhaustive dissertation on batteries.  It is meant to give some hints and tips on using your house batteries in your D.  Caution - batteries contain hazardous materials and can explode! If you do not know what you are doing, get professional help!
All of our D's contain two wet cell, automotive 12 volt chassis batteries that are wired in parallel and at least two wet cell deep-cycle 6 volt batteries that are wired in series to provide 12 volts to power the "house" portion of the D.  Note that wiring batteries in parallel keeps the voltage the same, but doubles the current capacity.  Wiring batteries in series doubles the voltage, but the current capacity remains the same.  When you connect batteries together in series or parallel, the MUST be the same voltage (6 volt or 12 volt) and the same current capacity AND they must be the same type (automotive, marine or deep-cycle) AND the same construction (wet/flooded cell, AGM, or gel).  Without going into a technical discussion, you will have big trouble if you don't follow these rules!
So, let's assume that your D has only two house batteries and you want to add two more.  BE SURE that the two batteries you add are two golf cart, deep cycle batteries - usually T-105.  Trojan is a good brand - avoid cheap substitutes at some of the automotive discount stores.
USING YOU BATTERIES
For maximum house battery life, it is essential that they be maintained properly.  Check the electrolyte level frequently (especially in hot weather and when boondocking) and NEVER use tap water - use DISTILLED water.
NEVER discharge your batteries below 50% capacity if you can avoid it.  Use the table below to determine your battery's state of charge.
If your battery has been charging, turn off the inverter and let it sit for at least two hours.  Also, the voltage listed is open-circuit voltage - that is, the battery has no dc load.  For practical purposes, just turn off everything you can and measure the dc voltage across two 6 volt batteries.  The voltage you measure will be slightly lower than reality, but you will avoid the possibly dangerous situation of disconnecting your battery bank.
Invest in a good temperature-compensated hydrometer or a good 3.5 digit digital voltmeter with an accuracy of at least 5%.  Also, note that the table above is accurate at 70 degrees F.  However, the main point here is to AVOID discharging you batteries below 50% to maximize your battery life.  Discharging below 40% - 20% counts as a "deep cycle" - and your batteries only have about 180 of these in them.
CHARGING YOUR BATTERIES
Once you have used your batteries, don't let them sit in a partially discharged state.  Charge them immediately!
Last, the 5 watt solar panel on the front air conditioner on our D is virtually useless.  It will NOT charge your batteries!
Typical golf-cart, lead-acid batteries typically provide the least cost per amp-hour by a wide margin.   However, if you ever switch to another type battery - such as a gel cell or AGM - you MUST set your Heart charger for your new type battery.  Refer to your Heart Interface owners manual for the specific procedure to your inverter/charger.
There are many excellent articles on the Net on batteries if you want to learn more!