Hi and welcome. This is a bus site, true...but many of us have other passions as well. We have an FL60 which was an ambulance which we converted and registered as an RV. Ours has a window, but no walk/crawl-through passage. I'm not exactly clear on what you mean by "have to" have a passage...it's not required, as far as I know, but it is certainly nice to have. In fact, that's the only thing about that rig which isn't quite perfect in my eyes...and we've got long-term plans to put a larger passage there.
You're welcome here, but you might find more precisely relevant info on the Expedition Portal Forum. There's a pretty active group of folks there who have medium duty ambulances, which are based on the same kind of frame as your Freightliner.
https://expeditionportal.com/forum/