There are a couple of companies that used to convert ambulances to 4 wheel drive. Once the F-450 chassis came out, most who wanted a 4X4 ambulance went with that chassis, instead of converting the E-350 chassis to 4x4 at a cost of around $25,000.
I have not seen any 4X4 ambulances based on GM vans or after about model year 2002 on the E-series either.
I think that Bigfoot did put in a roll cage at the factory both front and rear, built into the walls of the RV, so you barely know it is there, unless you are reading the manufactures data on the coach.
I always liked the "Scotty" brand RV, it was typically based on a diesel Dodge 3500 dually pickup. To bad they went out of business a few years ago. You could look up that brand on RvSearch.com.
Good luck on your search!
Fred.