I have used the Automatic Battery Conditioner made by Airflow on little used classic cars for years and can vouch that they work, I always have fully charged batteries even after months of not using the car. They also do a 24V intelligent charger which I use on the aircraft. Prices are good compared to most too.
The problem with using the external power plug is that it needs a constant live supply to energize the solenoid, that's why there are three holes in the plug!
Airflow is at
Air-Flow specialise in car care products, car covers, air chambers and car accessories.