Subscribed...are you talking 5er toy hauler tractor or Super C style toy hauler motorhome?
If the later, I was thinking of doing this exact build with a FL70 Box truck until we found our Outlaw.
For a motorhome, I would offer that the wheelbase would be around the standard 242" range used by many motorized toyhaulers (see our Outlaw, pictured below is 242"). They keep the wheelbase short(er) and drop the frame about a foot, right behind the rear axle for the garage floor to be lower to aid in loading and unloading with a ramp vs. a Tommylift.
The standard flatfloor box is fine too, as long as you are planning to use a Tommylift because a ramp gets real steep if you don't drop the rear frame.
Kim and Steve, Mustang LCDR (Ret), '07 Damon Outlaw #1193
I have seen gross intolerance shown in support of tolerance, Samuel Coleridge
WE LOVE OUR OUTLAW RV