Download google sketchup for free.
YouTube mastersketchup and watch the tutorials for the basics.
For cutting lists the secret is to make each piece a "group" by selecting it right click make group.
When you build the model don't draw it but assemble it. So say you are making a trailer that is 2400mm long by 1500mm wide. you would make a side rail that is 50mm wide, 75mm deep and then 2300 long. Group it, copy it and place them side by side.
Then make the end pieces 1500mm x 50 x 75 and do the same.
Connect them and you have the chassis. Select them all and make a group.
People have trouble with sketchup when they try and sculpt it from blocks. So they draw a 7x4 box for the trailer and start cutting bits out.
If you pm me I can make a start on the chassis for you and email the sketch to work on.