Unless you are building on a used truck, there is no need to stretch a chassis. Most all of the truck manufacturers offer a chassis combination, (single axle, tandem etc.) that will fit your build. I think that Freightliner is a little more flexible when it comes to custom orders, but the others want to get in on the truck conversion market also.
I know Mack has supplied several units for conversion in the last two years, they make one boss looking motorhome.