All interesting stuff about two batteries and solar panels etc but if you are using your boat every couple of weeks or so your battery should be more than man enough to keep the bilge pump running to get rain water out (and I know about it raining more up north
I think you may have either an unrelated electrical problem causing the battery to discharge or possibly that the boat is flooding through the self bailers. Float swithches once they have been around a bit are notorious for electrical leakage.
I assume (dangerous, I know) that you have the round white plastic self bailers with rubber non return flaps. If these get dirty or the rubber hardens a bit they will not seal properly. The bilge pump will then be operating much more often and will flatten the battery.
My Scorpion has these fitted but also has some rubber bungs to put in from the inside when leaving the boat unattended.
I hope this helps - there must be a simple solution!