Chuck,
I actually passed my federal FEMA CERT training which included fire classifications. Based on my class notes: There's a variety of metals which qualify for a Class-D extinguisher. Sodium, potassium, uranium, lithium, plutonium, calcium, magnesium and titanium. Hopefully noone has any Uranium or Plutonium in their bus. Likely magnesium or titanium are the common metals we might find, but I'm not aware of any, other than my wedding ring (titanium). Lithium is found in Electric vehicles, but I think uncommon in our large coaches. Of course a random flame wont ignite these metals, it takes significant heat with a good fuel source to get them started. Once started, grab your loved ones, some marshmallows and step back, very hard to extinguish.
It's my understanding that Cold Fire extinguishers also do not work on Class-D fires.
My belief is that Class-D fires are a small risk not worth worrying about. A good collection of Class- A,B,C extinguishers is sufficient.