In my whole career, I've only designed HVAC dutwork for two small office buildings, and never for a travel trailer, so don't take this as gospel without independent confirmationů
Your concept should work, with a few modifications. You don't really need a plenum if you've got a main duct. You just need a transition section, that matches the furnace shape on one end and the ductwork shape on the other. In general, the main ductwork cross-sectional area should be about the same size as the furnace output, to avoid either accelerating the flow (causing increased noise) or creating back-pressure that would reduce furnace efficiency.
If your furnace is in the middle of the trailer, then the furnace transition section goes straight to a manual diverter valve that splits the flow between two main ducts, one running in each direction. This lets you use smaller main ducts, by the way, since each only handles part of the total furnace output flow.
In a main duct, you will want manual diverter valves for each branch duct, so that you can adjust the volume of flow going to each branch. This procedure is called "balancing" the HVAC system, and only needs to be done once, when you test the system after installation.
If you want some of the heat to go to the belly pan, you will have one branch duct that terminates in the belly pan. But remember, anyplace that you're blowing hot air, there has to be someplace for cold air to leave, that will handle an equal flow volume. Make sure there is someplace for air blown into the belly pan to get back out, on the other side of the tanks from where the duct is.
Now for the numbers:
In the main duct, you want the flow velocity to be between 780 and 1380 feet per minute. Based on the cubic feet per minute that your furnace produces, that lets you determine the size of the main duct you will need. Remember, if you have two main ducts running in opposite directions, you will use the total square inches of BOTH main ducts to derive your ft/min flow velocity from cu.ft./min flow volume.
In each branch duct, you want the flow velocity to be between 200 ft./min. and 590 ft./min. The slower the flow, the quieter it will be, and the less of a breeze you will feel by the duct. But remember you're still pumping the same cubic feet of heated air even at a lower velocity.
The floor registers should have valves to close them off. This not only allows you to regulate the flow in real time (unlike the diverter valves or check valves, which are set once when you balance the system and left alone thereafter) but also allows you to close the ducts in summer when the furnace is not in use.
Since your trailer moves, and vibration can cause the manually-set diverter valves and check valves to vibrate to a different position, safety-wire the adjustment levers in place once they're set the way you want them.
Engineering: Finding complex solutions to simple problems you didn't even know you had.