IF you're going to boondock much at all you need 2 batteries.
What do you have now???
One 12v "crank/deep cycle?
Two 6v deep cycle??
There is a huge difference in storage capacity from the top-
One 12v all the way down to two 6v.
Get a decent digital VOM (volt ohm meter) and watch your battery.
When it gets down to 12v it's about 50% or a tad less charge remaining.
For best longest life try not to run your battery(ies) down below 50%.
We typically can go 5 days on two 12v batteries.
We'll watch a little TV using a cigarette socket inverter.
Maybe watch one DVD.
We use lights sparingly and we have a thinlite florescent over our
table for eating and reading. It uses about as much power as a single
12v incandescent bulb but puts out much more light.
We also have a couple LEDs that we use for "walk around lighting" when
it gets dark. These use only a fraction of one single incandescent bulb.
We'll typically pump a full tank of water in that time also.
This also includes a few hours per night of a maxxair ceiling exhaust fan
running on low speed.
Get a meter, watch your battery and see how you're doing.
As mentioned, you can plug in your truck to get a boost if you need
something to work like a keyless entry or jack or awning.
Read this FAQ on batteries--