The longest we have gone out boondocking is 5 days, but we made the five days with three kids that go in and out none stop and sometimes leave the old school light bulbs on. All on one battery, no generator. We did have a spare battery in the bed of the pick up just in case. Their are some advantages to having the vintage airstream..no amenities to suck the battery dry
We do do all our cooking out doors and hang out outside unless its bucketing down rain so I am sure that helps lessen the power consumption.
We've never actually ever gone plug in camping so having energy efficient habits deeply engrained helps. We have just switched the trailer over to all LED interior bulbs so we should be able to do longer in the future