I have had two vans, a Ford and a Dodge, and I am having trouble trying to remember WHY I would have to stand, other than getting in and out. I sat to cook inside on a propane stove -- the bed platform was eight feet long and 38" wide to fit a twin mattress. The mattress was pushed nearer the back door, and the bare 20-24" part of the platform was the end behind the driver's seat. The bare platform was were I used the propane stove (a 3-legged Grasshopper). I sat on the bed and stirred the pot when I was cooking inside.
I took birdbaths with a baking soda solution, sitting on a bath towel on the bed, and washed while sitting. I laid on the bed to pull my pants up. I didn't do any calisthenics or dancing in the van. If I needed something in the storage boxes under the bed platform, I either sat on the bed and pulled one out at a time, or stood outside and leaned in, pulling them out. I read or sewed or did stuff sitting on the bed or in the front passenger seat.
What is it that you plan to be doing that requires you to stand inside the van?