Using a volt meter check the voltage at your batteries. Should be 12.6+ volts at 100% with healthy battery. 12.06v at 50% charge level and 11.9 at 40% level. Generally speaking the batteries will last the longest if they are not discharged by more than 50% (flooded lead acid type can be discharged more than a AGM or gel type).
Easy enough to see what it takes to take the batteries down to where they show 12.1 volts and use that as your base. You can also see how long it takes with AC to recharge them to the maxium voltage they are able to provide.
On an ongoing basis installing a Trimetric or similar charge meter will make it easy to monitor what is going on throughout the day or a trip. Well worth the cost of buying and installing one.
Best book by far for general RV questions like this is the book ($11.32) "The Complete Book of Boondock RVing: Camping Off the Beaten Path" by Bill Moeller. Easy to read and extremely informative and directly applicable to RV users.
