I'm going to throw out the possibly controversial statement that no trailer has ever been more suitable for lengthy boondocking than the ones made now.
The 12 volt
technology available before around 1970 was worthless. Univolts then were junk compared to what people complain about today. Batteries were lower capacity, shorter lived, something you kinda planned on replacing every year as part of your seasonal maintenance, if you planned on using them.
In 1970 if you were serious about boondocking you got a Humphery gaslight, which is still an alternative, but with modern 12 volt
fluorescent ballasts, converters, and battery technology, it isn't really necessary.
In 1970 you would have had your B&W 12 volt
television which draw 8 amps or so. Maybe leave the engine running in the tow vehicle for a while to juice up the batteries, although with the electromechanical voltage regulator on the charging system, even if you were lucky enough to have an alternator, that might not do much.
Go back far enough and you get a panel ray heater that doesn't require electric, but, those have their limitations too, and use indoor air as a source for combustion.
Building on what 2air is saying, the classic trailers come with two batteries and with more efficient lighting. In the summer I don't run out of electricity while boondocking. In the winter, well, it's a matter of proper use. I see the furnace as something to be run morning and evening in a boondocking situation. Which, after all, is the way things were in stick houses before central heat.
Over time I have come to appreciate the fluorescent lighting in the classic more and more. It lacks the visual bling of the halogens in the CCD and the high tech geeky bling of the LEDs. But, in reality, the lumens per watt is better than either, and there are no problems with heat, and the diffuse, even light is great.