It looks like a four foot space, not too hard to find behind the axle of a C or a short wheelbase small A. In a B there will be a little more to finding a space for waste tanks, depending on chassis and fuel tank and filler location. I see that configuration fairly often in small TT's, only for those it is in a rear corner, and they often put fresh water tank at the other end for balance (which is kind of screwy if you think about weight distribution changing as the water gets used).
That would be the Axis 24.1, 25'6" long, 94" wide, on a stretched E-350 bare chassis. Most Cs are somewhat longer, might be 8" wider, and can carry another 2000 pounds than this thing Thor calls a RUV.