Two suggestions.
First the Progressive Dynamics 9200 series of converters (150-250 dollars refurb) are designed for the GC-2 pairs or other 11 volt flooded wet batteries.
For one pair you can get any size up to 70 amps but for re-charging between trips. THE SMALLEST IS BETTER.
For about 100 dollars (At auto zone) I have a lovely Deltran Battery Tender.
The link below is NOT the precise model I have. the one I have does "Tender mode" (A couple amps) "Boost" (20 amps) and "Jump Start" (70 or 75 amps not sure witch) and an app on my smart phone connects via Wi-Fi and controls it.
http://products.batterytender.com/Chargers/Power-Plus.html
