Sketchup is exactly what you should be using. It's free and easy to get started with, with more powerful features available.
Check out the 3D Warehouse - there's a good chance someone has already modeled something you need
(here's a sample search of the word "camper")
Video tutorials from them too:
http://www.sketchup.com/learn/videos?playlist=58
They want you to love this program so much that you decide to buy the pro version, so it really is great. I'll never build anything again without modeling it first.