ours has an enclosed underbelly. Here's my opinion on the reason and marketing of enclosed underbellies. Reason for it has almost nothing to to with winter camping, it's primarily done because it's easier to build the trailer. Instead of wiring etc in the roof and paying attention to plumbing, the put the floor on the frame, flip the trailer frame over, run all the wiring and plumbing etc and don't do much for attachment, then put on the covering, flip the frame right side up and finish building.
So....... then the mktg dept was told come up with an ad campain. They came up with the 3 or 4 season idea with heated underbelly. In most cases the heat is the radiant heat from the ducting, no direct heat vent.
In my case the ducting was not great along with a few things I wanted to do so I pulled the covering, did the upgrades, which would have been much harder with an open underbelly due to access to some wiring etc that would have been in the ceiling, closed it back up and on two of the heat ducts drilled a 1/2" hole in the side of the vent in the floor to get heat underneath.
Yes, I like the enclosed underbelly but for many many people there is not a real advantage.