I took Bill's advice and bought a Guest trickle charger for my 2720SL. Since my batteries are in the back compartment, I routed some new wires for the charger directly from the battery down through the floor and into the bumper compartment and then into a small tuperware container. Once in the container, I terminated them with a a car alternator connector, and soldered the mating connector to the battery charger.
Now when I want to charge the batteries and the unit is closed, I just pop open the bumper compartment, take the lid off the tuperware container, and hook up the charger.
BTW, the best deal by far on a pretty decent Guest charger was from a company I've never heard of: BriJo Marine....$85 + free ground shipping (or $6 for 2-day air) for the Guest 16061, which is a 6 amp/12v 1-bank charger. Talked with the guy on the phone, and it was here 2 days later.
There are different settings on the charger for acid, gel, and AGM, and it has an LED indicator to tell you which of the 3-stages of the charging process it's in (bulb, absorption, or float). Very easy to use...been very pleased.
BriJo's link is here:
http://www.brijoinc.com/marine/index...product_id=490
Dave