A small diesel pusher that handles well is going to be hard to find, since having the engine and transmission behind the rear axle requires a certain amount of rear overhang (with a good bit of weight). If the overall chassis is too short, this overhang becomes relatively larger and cause handling and balance problems.
That's not to say this is an impossible problem to work around; a traverse engine and transaxle design similar to a front wheel drive car, for instance, could allow for a rear engine with relatively minimal rear overhang. I rather doubt anyone would make such a unit for what is relatively speaking a very tiny market (small diesel pusher motorhomes would never be common vehicles, at least in comparison to larger ones, or to medium and heavy duty trucks, or to vans or cars....)
