I have a Transcraft Tl-2000 semi trailer, which is in a remarkably
decent condition, considering it is 17 years old. It has superficial
surface rust, but no through holes or big pieces or rust falling off.

It was owned by a construction cmpany prior to me buying it at
auction, and it was probably not used much during winter.

Well I will need to use it during winter, so, my question, how to rust
proof it cheaply. (it cost me only $1.5k plus buyer premium). Say, can
I spray some oil on the frame every couple of months? Would that work
satisfactorily?

i