Hi,
Like stated above, there is a few companies that convert E-350 vans for use as ambulances for anyone interested in spending $25,000 for the conversion. A ambulance company might convert some of it's fleet, just to have one ready for use in deep snow and other winter conditions. What that company does is install the front end parts of a 4 wheel drive pickup on the van. It takes a lot of time and parts, and will not effect the rest of the van's warranty.
If you don't want to spend that much on a van based class C, you might try looking for a 4 wheel drive version that has already been built. Several RV's where built on the F-550 truck chassis, just as easy to have one built in 4 wheel drive, though many new buyers are not interested in spending a extra $2,500 for the 4 wheel drive option. Ambulance companies do make many of their units on the F-350 and F-450 4 wheel drive cab and chassis trucks. I don't know if they still convert the vans, due mainly to the cost of conversion, and ease to use a F-450 instead.
Even the 5500 series GMC trucks had a 4 wheel drive option. However while 100 RV might be built on the GMC 5500 series truck chassis, only 1 or 2 might be 4 wheel drive option. GMC stopped building the 5500 series chassis back in 2007, so the newest RV's where probably finished in 2008 or 2009 at the latest.
So you might find everal manufactures that built 4 wheel drive chassis. They built them mainly on truck chassis, with some installing a 4 wheel drive front axle on the E-350 and E-450 chassis.
Scotty was one camper manufacture to make them on a 2 WD and 4 WD Dodge chassis. The only problem is many customers also wanted the 5.9L inline 6 diesel, that was a heavy option, leaving very little cargo rating before exceeding the truck GVWR. So the 400 pound 4 wheel drive option was not very popular.
Tiger (mentioned above) was also a light weight truck camper mounted to a 2500 series 4 wheel drive pickup. They kept things very light weight. But they have not been making new units for many years.
I was driving down the freeway last week, and saw what looked like a F-550 based motorhome in a new RV lot. I looked, and found it to be a 30' long Four Winds, built this year. It was 2 WD, and no reason a 4 wheel option could not be used as well. It has a 2800 pound cargo rating, including the weight of water in the tanks, so it is not as generous as one might think. If you have a 700 pound hitch weight, 500 pounds of water, and add 100 pounds food, cooking items, and add clothing, 500 pounds of passengers, you can see it all gets consumed quickly.
Good luck finding something.
Fred.