If you're running a BLS agency, or have a BLS fleet, then vans are perfectly reasonable. For ALS services I think boxes should be the standard.
Many years ago, I discussed this very concept with my operations manager. We were a ALS agency that had boxes for BLS 911, ALS 911, and RN CCT/911, vans for BLS IFT, and were beginning to transition our ALS crews to SUVs (by state regulations, BLS transported everyone, if the patient was ALS, they hopped into the BLS truck; exceptions were made if BLS and ALS were the same agency, but that was not the norm).
I said why not put ALS in a van? for the rare times that you did need to transport, you had the ability to, and they were cheaper than a box, considering 98% of the time if you were treating, the truck was functioning as a flycar, in transports that lasted less than 30 mintues. now compare than to BLS IFT: with my 6'3" frame, sitting on the bench seat (which was curved upwards) for long distance transports of over an hour one way, can you see why many of us needed to stretch after the run? But apparently he wouldn't even consider it, because he wasn't going to put ALS in a van (likely due to the objections from the providers).
I've worked cardiac arrests in vans (and had one person who coded in transit), they are tight quarters when you have extra people in the back. But there are still plenty of places in the US that run with a single person in the back for all calls, or 2 at the most (a provider and a trainee).
With scoop and run becoming more of a thing of the past, how many people do we really need in the back of a truck?