I think someone already figured this out in earlier comments, I just found this out now. For anyone having their trailers disappearing, check if you have Week One mod, it treats them as wrecks and actively removes them pre-pademic. You can disable this in BWOscheduler.lua:

if BWOScheduler.WorldAge < 90 then BWOScheduler.World.VehicleFixer = true

Change that to false and this will allow all wrecks to exist pre-pademic. This might or might not have consequences for NPC driven vehicle spawns FYI.

PS. I'm running the B42 ver for both.