As you have observed, there is a large price difference between something that is newer and something that is older. So the challenge seems to me to be to find that perfect combination of an "older" trailer that has been well kept (stored under a roof in an arid climate, and not used until it is worn out). There are things that will age, such as axles and door, window, and vent seals, but most of these things are relatively easily/cheaply fixed.
I find that there are two kinds of RV owners--the ones that use them routinely, and the ones that hardly use them at all. You might find a 30 year old trailer that has relatively little wear on it from the previous owner, and yet has been taken care of. This is the goal, but you may have to do a lot of tire-kicking to find that combination. What you get by buying something newer is ease of purchase--the risk of a trailer that is only a few years old being in bad shape is relatively low. The risk of a trailer that 30 year old has problems is high, so you have to spend a lot more time researching, inspecting, investigating, etc., to manage that risk.