In Microsoft's continuing quest to compete with Google, they bought Caligari. And to compete with Sketchup, they released Truespace 7.6 for free. This is very powerful 3D modeling and animation software. It exports to Microsoft Virtual Earth.
I have been using an older copy of Truespace for a while, and it works pretty well. It is harder to learn than Sketchup, but it can do much more. They have also released all of the video course for free, so it should be possible to figure it out.
It can be found at
http://www.caligari.com/
They do require registration to download.
I figure this will be good for modeling all of the folding type of campers that I like!