My 2 cents...............
A ramp is not needed. My reasons:
1) The rear door opening, while it looks big in the pictures it really is not all that big. To walk cones in or dolly them would really be a PITA.
2) Mounted above the rear door is the (working) rear AC unit. Just ask Stu how many times he hit his head while we were working on the bus. I know I did at least three times.
Once we get the title and plates on her then we need to have a day (or two) of layout planning and design. I agree with Jim, keeping as much stuff off the floor as possible would be ideal. We did do a test fit with a folding table and chairs and they did take a fair bit of room when setup.
As for dimensions we are looking at 12' long by 6' 6" wide by 5' 5" tall. I didn't get the rear wheel well or floor heater locations measured out.