I was planning on using something like this
but it wont fit underneath (gas tank in the way). Along that vein though, I dont know if you've ridden/pushed/walked big bikes up a ramp but it can be pretty sketchy. Nothing like having 600lbs of german engineering coming down on top of you from 3 feet off the ground
To mitigate this, I'd planned on getting a mini winch from Harbor Freight and mounting it to the ceiling on one of the ribs. Then I could just balance the bike instead of working the clutch, throttle, front brake, balance, and foot placement.
As I've abandoned the retractable ramp idea, the next though was a little more involved. I dont want loose ramps inside, they are a pain, they rattle, and they are always in the way. So I measured out the largest possible rectangle on the back of the bus, for mine its 56x84". I called a local trailer shop and they are making me a toy hauler ramp door for $600. I'll also have them cut and install it, another $4-600 depending on how long it takes them. Then I'll have a full width ramp that is also 12" taller than the emergency exit door that I was going to have to fab anyway to fit a bike through. The height of the door is also just long enough to have a decent slope for loading, even after the 6" lift that is going on next month. It'll be a little steep, ie you're not driving a jeep up it, but for moto's or an atv its fine.
Another awesome benefit of the toy hauler back end is that I'll be able to open it halfway and secure it with a chain (think prison bunk bed) and have an awesome patio/deck. There also several options for magnetic window screens made for the back of toy haulers which will make for fabulous ventilation with the bugs.
I'm in the final stages of the planning phase now and plan to start burning cash in earnest next month.