On my start batteries, I have something like what SDog suggests: a solar panel that delivers a trickle charge to the two batteries each through diodes and a small (1/2 ohm) resistor, to split the current. It does not depend on shore power. The panels are a little pricey, but once installed this setup works well without maintenance of any sort.
Not sure how well it would work here in DC. Haven't seen more than a couple hours of sun in weeks now......
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida