I too use Sketchup. It's a WONDERFUL tool. What I've found helpful is to color code your wall panels, so you can see even more clearly how to build it when the time comes next year. (IE, 4' = yellow, 3' = orange, 2' = pink, 1' = blue, emergency exits = green)
Definitely use the tutorials. Lots of tricks to learn. I've gotten to the point of having a building model (with the colored panels) and a 'props' model, where you sketch it up to sketchup props and place them in the virtual haunt. Halloween Horror Nights does this and it's VERY cool... I believe there is a youtube video about them... maybe...
Twisted Realms Haunted House