Our 2008 E450 came with a 5 year/60k mile power train warranty that includes the engine. I've never heard of a vehicle power train warranty that didn't include the engine. In my opinion, it's not a power train warranty if it doesn't include the engine.
As far as he VIN goes, the one time we ordered a RV, about 10 days after ordering it, we had the trailer VIN #. It wasn't delivered to the dealer for another 5 weeks or so. That was a Forest River Hybrid Travel Trailer (Rockwood Roo). You should be able to get the VIN number of the E450 and the serial of the house from the dealer a week or two after it has been ordered.
I would be concerned with a Dealership that didn't know the E450 warranty. I can't imagine it is the first Motorhome they have sold, so they should know the warranty. They are either really bad at their job or they are trying to trick you into buying an extended warranty.
-Michael