It is possible to order a chassis, have it sent to allrad, then shipped to the USA (north American spec). You could then have the RV bits added. That would be a loooong wait though. Plus some 25k$ in costs.
You can order the 3500 cab/chassis with 4x4 since mid 2018. But its a special order. Since most coach builders don't order it, and because there is a strict limit on 4x4s sold in the USA, they are really rare. (this is due to oberaingers diff production limits).
If you can find a wrecked 4x4 sprinter with the right diff ratio, you can transfer the front axle assembly over, and use a custom Tcase. Mostly bolt-on.
There was a company that used MB factory front knuckles, hubs, subframe, spring, and CV axles. They used a custom diff and Tcase. This cuts out the really hard to fine MB front diff, and allows for a beefy front diff and Tcase (like atlas). It was something like 20k. There is no reason that could not be done to a cab/chassis. I will see if I can find the website.