SST, remember that the legal limit, I believe is 45' overall length in most if not all states. You will find that most motorhome manufacturers only make a 34' box as their largest. Look for Elburn Bill's pictures and floorplan on this forum, he has a very nice Kingsley, I've seen it and I believe it is longer than 45'. Some of the coach builders, like Kingsley, Showhauler, NRC and United Specialties will build your custom plan (within reason).
Bob