You never said what size (or capacity) battery you are using. With the electronics load I have (similar to yours) I use a Group 31 dual purpose battery. Gone are the days uf using low capacity Group 27 batteries if you want to survive those loads. Group 31 is typically the largest practical physical size available for boating. As noted, you need to put units you aren't looking at in low-power mode. This can easily cut their power consumption by more than 50%. Wake-up is almost instantaneous for HDS, so there's no aggravation with using this approach to save power. An HDS 12 can draw more power than an aerator pump, BTW.
New batteries are not best simply because they are new. One with insufficient capacity will fail, and replacing it will just get round 2 of the same. For starters, if you are using flooded cell lead acid technology (typical) you need a good hydrometer. This will allow you to insure that your charger is doing what it is supposed to in the first place. Here's an example of a good one:
Battery Hydrometer To use it properly, let the charger run a couple of days. Then, unplug it overnight to let the battery rest at least 12 hours. Measuring the specific gravity after this rest gives the most accurate result. The hydrometer will have a chart of what to expect, or you can look at the detailed battery specs from the manufacturer. If the battery isn't at the proper specific gravity, you have a bad battery or a bad charger.
I know you said you keep the boat garaged with charger running. But, sometimes boats have unrealized loads that contribute to battery drain. Example: Navico radio wired to be energized anytime main +12V is present, even if you aren't listening to anything. I've encountered a number of boats that had things wired and consuming power when the owner was not aware. This can be checked easily using a small current ammeter (a function of most DVM tools) if you have a friend with a bit of electrical knowledge.
