Our 3300RLB also does not have a heating vent in the master bedroom. Here is my guess as to why.
1. Too long of distance from the furnace to be effective. Even the air flow in the bathroom is low and not that warm.
2. Not very easy to find a good routing path to the bedroom for a vent line. It would block the bottom storage area more than the one to the bath room does already.
3. Heat is supposed to rise, so perhaps the thought is the heated air in the rest of the trailer will migrate to the higher bedroom.
We've learned to just live with it. We do notice a heating effect due to #3 above, but it is a slow effect.