arthur, the ammeter on your instrument panel is most likely a 0-25 amp meter. As such, it has a built in shunt and is wired in series to the common terminal of your 1-2-B switch. When current is being drawn, it shows how much at that point in time. It is NOT a "battery fuel gage" because it only shows momentary consumption, not how much "fuel" (amp hours) is left.
For instance, when my fridge turns on, it use about 5 amps, so when the ammeter shows 5 and I have no lights or anything else running, I know the fridge is working. The fridge works about 50% of the time, cycling on and off. So, 5 amps X 24 hours X 50% = 60 amp hours a day. See my Energy Budget post (
http://c34.org/bbs/index.php?topic=3976.0).
Battery "Capacity" -- and need for recharging - only an amp hour meter, like a Link 10, will tell you that, unless you can do the math in your head about consumption compared to capacity. If you have a 100 ah house battery (you did say you have a single battery) then you only have 50 amp hours available to use, which is one day use or less of the fridge turned on all the time without using any other 12V loads. That assumes that your battery is fully charged. It's capacity of battery (bank) times one half less amp hours taken out.
Battery Condition: The ammeter then does NOT tell you ANYTHING about the condition of your batteries. The ONLY way to know that is to have a multimeter voltage measurement device and a hydrometer which reads specific gravity of the acid in the battery cells (assuming you have wet cells).
If you haven't yet, I highly recommend that you purchase, beg, borrow or lend from a library Nigel Calder's Boatowner's Manual for Mechanical and Electrical Systems. It's on its 3rd edition, but if you find a 2nd edition it'll most likely be less expensive and cover all you need to know.
If you are anchoring out a lot, and like to do that, you NEED to add more batteries to increase your house bank. Others will say you need a generator, a higher output alternator and a regulator to run it, or solar panels. Many different ways to skin the cat.