never worried about our water line to the ice make, and it's never been any problem... whether in storage or not.
as for heating the 'wet bay', yes, we sometimes stick a small electric 'auto on' heater in there, and run an extension cord from another outlet on the campground pedestal, or from the other side of the coach in the outdoor storage bay where there IS an outlet. You have to be careful as to tripping breakers if there is more than one on each circuit.
we run electric heaters in our coach, too, but if the furnace kicks on only several times throughout the night, your basement wetbay area should probably still whether the temps with few issues. Most folks worry a little much about low temps, as in these coaches, most everything is enclosed within the RV, not exposed outside. The exterior water hose to your spigot would be the first thing to freeze - simply turning off the spigot and unhooking your coach end of the hose for the overnight hours is the easiest way to protect it.
'14 Thor Palazzo 33.3 compact diesel 100k+miles since '14... US, Alaska, Canada...
AND a '14 Gulfstream Amerlite
AND a '07 ForestRIver Rockwood