First off it's impossible to overcharge your trailer battery
from the charge line. When was the last time your cranking battery was "over charged"? The safest way to charge the house battery while under tow is with the charge line from the tow vehicle.
If your Scamp
has a "converter" it already has a battery charger. If don't have a converter then a simple maintainer will work just fine.
If you're going to connected to 120 most of the time no need to worry about charging the battery at all since the converter should take care of your 12 volt needs. Boondocking
then other methods like a solar panel