There is a rectangular heat duct tied to the propane furnace that runs the length of the floor from the duct in the front of the rv to the duct in the back, with floor duct in the middle and a 3" hose coming up thru the floor that leads to a duct in the bathroom. There is a 3" hole in the bottom of that duct that puts heat into the basement only when the propane furnace is running. The return air side of the furnace draws the air back in from the basement. Having the water heater on also will put some heat into the basement.
If you have your water heater on and are running the furnace at a "normal" temperature such as 60 degrees or more it should keep your water pipes, water tank, and holding tanks from freezing at temps in the mid to high 20s. Below that beware. If you heat with electric heaters in the living area of your RV and not the propane furnace then the basement will need some other heat source to protect things in there from freezing.
Personally I added insulation and closed air gaps below the floor on my Vista to try to help
Randy - Manhattan, Kansas
2015 Vista 27N
2016 Honda HR-V