They seem to put the duct anyplace it will fit. My class C has the furnace under the stove, so the duct goes through the cabinet under the sink and into the bench for the dinette. If you go under the floor, make sure it is well insulated.
When I build my new truck, I'm thinking about running ducts through the floor. You would probably want to talk to an HVAC guy to see how much airflow you will need. If I wasn't going to carpet all the floor, I probably would have experimented with radiant floor heat. Just install an extra diesel fired hot water tank and a pump.
For me, AC is going to be more important than heat. I'm considering going under the floor there too. If I can find some auxillery units like they make for sleepers at a reasonable price.