Thank you for your response. I have the same concern, that's why I am asking here.
"The ideal humidity for rust-free storage of automobiles is between 50 and 55 % RH (relative humidity)"
I'm not sure what the situation in an enclosed trailer is throughout the year.
We use a couple of steel shipping containers at our archery club for storing the target bosses when not in use. The steel parts (pins, hooks, etc) haven't rusted badly but they do show rust so I wouldn't rule it out on a car. The containers themselves aren't that bad although having said that we found a hole in the roof section last year which had rusted through.
The other aspect is that even in our Lincolnshire climate the internal temperature varies quite a lot. It's basically a big steel box and in direct sunlight with the doors closed it gets quite toasty. They are never air tight so add in some atmospheric moisture if it's been closed for a long time and I can see paint problems - those little blisters - on the horizon.
I can however see the attraction for what you're facing. All I'd say is to try and get some ventilation and don't leave it shut up over for weeks over winter ?
Brian