I am one of those that use the internal charger to keep my battery up.
I left the TT plugged into the 120 volt outlet in my garage with an adapter from the TT power cord to the 15 amp wall socket.
It was plugged in from November to March. Since the internal coverter delivers charges the battery while we are camping all summer, I did not see why it could not do it in the winter.
The newer chargers are designed to sense the amount of charge needed by the battery and adjust accordingly.
Though limited to 15 or 20 amps I have used the adapter to power the TT from non campground sources.