just a suggestion, if it was me and I was going to add walls to a trailer again, I would buy some foam sheet insulation the thickness of the wall framing, so when you add wall panels they will fit flush tight to things.
as for material to use on the walls, the last time I did a trailer I got lucky and found a guy on Craigslist
selling white 5 ft x8 ft sheets of plastic,(20 bucks a sheet only too)
so ,that's what I used and came out pretty nice!
as fit perfect from top of kicker to ceiling!
I have seen them for sale since ,m so might be worth looking around for like sheeting, seems they sell it used in many locations? the guy I got mine off has it several time a yr for the past 10 yrs now!
Other wise, I think if I planned on keeping trailer a long time, I would invest in the proper wall material, something like the SuperLite RV wall panels or talk to a few other trailer OEM's and ask what they use and see what they would charge to sell you enough to do your trailer yourself!
this way you have something that will last a long time, and not have worries about cold temps and things cracking from cold air, not all plastic or composite l sheeting is able to handle very cold temps and cracks easy
after all a trailer is more of an investment than a single use buy item
and at current trends, if they stay this way, trailers are worth more than you paid for them, pretty quickly! new ones seem to go up almost weekly anymore HAHA!