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).
'18 Renegade 45'
Volvo Twin Screw, 500 hp I-Shift 12 Speed
30' Renegade Stacker Trailer