Vapor barrier - inside or outside.

Yup, there's some debate, you'll get some outside votes. But not from me, hehehe.

Sorry, can't explain in any fewer words, I tried. It's a problem I have

The Dept. of Energy says:
"INSTALLING VAPOR DIFFUSION RETARDERS FOR NEW CONSTRUCTION
In mild climates, materials like painted gypsum wallboard and plaster wall coatings may be enough to impede moisture diffusion. In more extreme climates, higher-perm vapor diffusion retarders are advisable for new construction. They perform best when installed closest to the warm side of a structural assembly -- toward the interior of the building in cold climates and toward the exterior in hot/wet climates."
energy.gov/energysaver/articles/vapor-barriers-or-vapor-diffusion-retarders

Like Bill said, you put it on the warm side of the insulation, to prevent moisture from getting past the insulation to the cold side and condensing. So for real winter camping, with snow 'n' sh- I mean, snow 'n' crap, it should go on the inside. For always camping in a rain forest with kick-ass AC, it should go on the outside. For everything in between, eh, you probably don't absolutely need one. They didn't use them when they built these beasts, and it took 40 years for them to rot out, and it was almost always from leaks, not condensation. I really don't think it's much of an issue anyway for most campers, who might only do a weekend or two in the winter. Any moisture that builds up will evaporate or sublimate before it sits long enough to rot or grow mold. The ones I've seen with the nastiest condensation damage (generally wet paneling, mold, funky smells, never drying out inside) have been ones used for hunting in cold weather, with a bunch of wet men cooking chili and breathing, belching and farting too much for days on end. But anything you can do to make a camper more tight is an increase in energy efficiency, so why not?

IMO if you split your camping equally between Alaska and the Amazon, the bigger chance of cooling any moist air in the wall below its dew point comes in Alaska, because of the combination of being in a small space with the windows closed (to keep from dying and being eaten by bears), with a larger temperature differential in winter than summer (30-40 degree drop between summer heat and AC, vs. 50-70 in snow country, or even more if you're really hardcore), with wet clothing hanging everywhere, a wet floor, breathing with reduced ventilation, steamy showers if you've got one, and cooking steam/exhaust. If you have any unvented propane appliances beyond the stove, like gas lights, or a catalytic heater, they put even more moisture in the air as a product of burning propane.

FWIW, I didn't use a separate barrier, I used foil-covered Tuff-R insulation (has the highest R-rating I could find), and then taped all the seams and over the wooden frame with aluminum tape, so no moisture can get past the back of the birch. Where the 110v wiring goes through the walls and behind outlets and other small weird spaces like the very narrow gaps between wooden frame and window frame, around the city water fitting, drains and propane line entrances, I used spray cans of Great Stuff, and covered it with tape too. I've only camped down to 14 F so far, and I haven't even finished insulating the front yet, but it's worked fine so far, even cooking without the range hood being reinstalled yet. I had it 75 degrees inside with my old Suburban furnace. The only condensation has been on the windows and on a couple of patches of bare aluminum that have no insulation yet. TBH, I think it serving as an air barrier is as important, if not more, than as a moisture barrier when it comes to cold weather. When the whole trailer's tight, you stay warmer on less fuel, and cracking a roof vent or a window is enough to keep dry in most cases.