You should look at the Eagle Cap 1160 and 1165, Host Everest and Mammoth and Lance 1172. These are largest truck campers in productions that are not custom one-off builds. Even you still decide to build your own, you may get design and layout ideas from these models.
I have my preference and have have made my choice, but take a look at all of them and see if any will work for you. Here is another thread for you to browse through showing some pictures of my choice even though we are still few a few weeks out from delivery:
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29117589.cfmWe're in a truck camper so I can get sufficient tow ratings and 4wd without going overly large with a Super C or diesel pusher or custom build.
Another design inspiration for you to look at are gooseneck fifth wheels with living quarters. They sit lower than most typical fifth wheels and the living quarters layout could be applied to a truck camper. These are available in 10-18' layouts and can incorporate slides if you wish.