In as much as EVERY RV configuration excels in some way(s) and is a compromise in others, that configuration sounds like a great option for that particular (very limited) use.
General limitations or turnoffs for everyone else who may not have the exact same wants, needs or preferences...
Can only haul 2 people comfortably, like you said. Non starter for long trips or families.
Doesn't tow well, compared to other options. That combo is a guaranteed pooch in all but flat towing down the road at low altitude with the wind at your back.
Overall comfort. A stripped down F350 reg chassis cab is not exactly what most people expect for passenger and creature comforts.
Cost/benefit. Most people use their daily driver pickups to tow their RVs. Dual use benefit. A personal box van, if not a rental scenario, is even more useless unless you're going to home depot. And the appearance thing and lack of practicality of using a 16' box van as a daily driver.
Appearance. If I'm moving stuff, I'm moving. If I'm on vacation, I don't want to look like a homeless person driving a Uhaul.
And, no, I've never seen any rental 6pack box vans. Gonna say those don't exist. If buying a crew cab chassis with box, refer to the 2 reasons above.
I applaud the ingenuity and yes it works, but there's a reason it's not a thing, at all, for recreational RVers. Although dirt bike race teams do it all the time. Different application, really.