Originally Posted by wa8yxm
Alas, this is NOT a good suggestion, since it does not allow you to direct warm air to the defroster vents.
But it is how I had to do it on a car, thankfully in the summer, a couple of times.
It's a little different when the outside air is below freezing. Any moisture inside condenses on the inside of the windshield and if the OAT is low enough the moisture freezes.
Anyway you look at it, replacing or having replaced a radiator or a heater core on an RV isn't a quick in and out thing.