Do i really need a TT with enclosed underbelly?

I've owned TT's both with and without underbelly cover, and find that without, the TT gets much colder at night that the one with underbelly. I was surprised at the difference. I'm using electric space heater, not the ducted propane heater, so whether the underbelly is heated or not is not a factor. So I probably would not buy an RV without underbelly cover in future.