The only 12 volt outlet in our TT is at the antenna amplifier.
We get about two days of toilet flushing/hand washing water and lights with our single 12 volt battery. If I need a third day I will use a set of jumpers from the truck and charge the TT battery for about 15 minutes of so.
In answer to the OP question, many TTs just have a 100 amp-hour battery. An incandescent bulb can draw about an amp. I measure my phantom loads at 1/3 amp. (with the TV antenna amp off.) So just having one lamp on, my battery would last 3 days. From that you subtract water pump time for flushes, washing, cooking and coffee making and two days is about right for us.
Recently we replaced our lights with LEDs but we also have multiple lights on. So, in reality if you want to spend more than a day or so boondocking you have to be set up for it or do without.
We seldom boondock, so 2 days serves us well. If you like to boondock there are a lot of folks on this board that can help.