There's some good advice here. I've been running my own business for about 13 years now. I have a lot of people (corporate types) come to me and ask my advice about going on their own. The most important thing to do is put together a basic business plan that factor in the costs of not only your product, but also things like time, insurance, licenses, taxes, etc.
I'm sure that you could make and sell some trailers, as I suspect there will be lots of people attempting to downsize their RVs in this economy. But building and selling a few trailers does not necessarily mean you can make a living at it. I'd start doing it on the side, building on nights and weekends. Use free advertising sources like Craigslist, carefully track your time and costs, and see where it goes from there.