How much solar do I need?

You and your friend need to do some reading on solar. Start with an energy potential use survey. That will determine necessary battery amps. Then...

Winter is tough on solar due to low angle and clouds. Cannot say an exact number any more than you can predict the weather. Best shot will be to supplement charging to reduce generator run time.

For travelling I would typically recommend to mount flat on the roof. However for winter in a fixed location a ground set up tilted to the sun is probably best. If you are in a shaded valley or in a bunch of trees it may not even be worth the effort and cost.

Otherwise I recommend 300 to 800 watts and a small 2000w generator. +1 to just start with a utility drop and focus on the project.

Maybe 300-400 watts with a generator for back up would be in the ball park.

Typing on a phone at Dr office, duplicate

Duplicate

X-2 on construction drop. It's what I did when I built in Mecklenburg county VA.
Another item you may want to consider is use of a lil buddy or a wave heater to save on fan operation. Yes, pay attention to safety with unvented heaters.
Also will help a lot if they skirt the trailer. Eliminating wind blowing under helps
A small Genny, even just a little 1000 watt unit to charge batteries on cloudy days will also be necessary.
Good luck.

Is the cabin they're building going to be on the grid, or off-grid?

If it's not off-grid, it would seem to me to make the most sense to get a temporary/construction electric panel set up ASAP and hook the trailer up to that. Insulating/enclosing the trailer (while still maintaining safety and preventing the buildup of CO, CO2, water vapor etc. from the combustion appliances) would also be a big help; naturally, any solar installation oughtn't be covered over with a roof or something for it to work.

There is 830w flat on the roof of our MH and I doubt we could do what the OP wants to do in winter due to so much overcast, but anyway---

You need the Latitude to calculate the optimum tilt up for the panels facing South, assuming no twirling. You need insolation tables for that location during those months to get an idea of how much sunshine vs overcast is likely. (Can get that on-line)

Pick a number for AH used in a day. Say 100, but could be more with the furnace on. Pick a number for time with sunshine on a sunny day there, say 5 hours. You need to average 20 amps, which would be about 600w worth of solar (300 would do 20 at high noon briefly then it is less the rest of the time, so pick a number--I pick 600--YMMV)

Now you still have all those overcast days in winter, so have a generator too. Absolute must! And a decent sized charger that can do 14.4 or so volts for the Bulk stage.

You have them coming and going from the RV, so if it is sunny while not there, solar can catch up a bit compared with the higher AH use when they are there. Luck of the draw if that does any good. Might.

All very chancy, and I don't think we could do that. Winter is not a good time for solar!