I used Google sketch-up and it worked fine. It takes a bit of getting used to the program. But once you get the hang of it, you can generate some good representations. I gave my design to the metal fab guys and they did a great job. I also used it to design an entertainment center for the cabinet shop guys to build for our new house. Can't beat the software's price.....free.
Good luck if you also want something that calculates loads, etc. I found nothing out there when I was looking six months ago, and have seen similar threads looking for similar to no avail. For this you need to rely on your gut and what others have done for similar tanks.