You should get at least 48 hours with a good RV battery
. Add up the various power consumptions of your lights
etc. Don't forget your carbon monoxide, propane
gas detectors etc. which will also always be drawing battery power. Divide the power consumption (amp/hour) requirements by the rated capacity of your RV battery. Now cut this number in half because you don't want to draw your battery down more than half way or you will greatly shorten its life.
Ten LED lights
will draw about the same battery power as one filament bulb so should be used if possible.